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Conversations on Chemistry, V. 1-2
by Jane Marcet
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EMILY.

The mere diminution of temperature, which they experience by the privation of animal heat, must, I should suppose, be sufficient to derange the order of attractions that existed during life.

MRS. B.

That is one of the causes, no doubt: but there are many other circumstances which prevent us from studying the nature of living animal substances. We must therefore, in a considerable degree, confine our researches to the phenomena of these compounds in their inanimate state.

These three kinds of animal matter, gelatine, albumen, and fibrine, form the basis of all the various parts of the animal system; either solid, as the skin, flesh, nerves, membranes, cartilages, and bones; or fluid, as blood, chyle, milk, mucus, the gastric and pancreatic juices, bile, perspiration, saliva, tears, &c.

CAROLINE.

Is it not surprising that so great a variety of substances, and so different in their nature, should yet all arise from so few materials, and from the same original elements?

MRS. B.

The difference in the nature of various bodies depends, as I have often observed to you, rather on their state of combination, than on the materials of which they are composed. Thus, in considering the chemical nature of the creation in a general point of view, we observe that it is throughout composed of a very small number of elements. But when we divide it into the three kingdoms, we find that, in the mineral, the combinations seem to result from the union of elements casually brought together; whilst in the vegetable and animal kingdoms, the attractions are peculiarly and regularly produced by appropriate organs, whose action depends on the vital principle. And we may further observe, that by means of certain spontaneous changes and decompositions, the elements of one kind of matter become subservient to the reproduction of another; so that the three kingdoms are intimately connected, and constantly contributing to the preservation of each other.

EMILY.

There is, however, one very considerable class of elements, which seems to be confined to the mineral kingdom: I mean metals.

MRS. B.

Not entirely; they are found, though in very minute quantities, both in the vegetable and animal kingdoms. A small portion of earths and sulphur enters also into the composition of organised bodies. Phosphorus, however, is almost entirely confined to the animal kingdom; and nitrogen, but with few exceptions, is extremely scarce in vegetables.

Let us now proceed to examine the nature of the three principal materials of the animal system.

Gelatine, or jelly, is the chief ingredient of skin, and of all the membranous parts of animals. It may be obtained from these substances, by means of boiling water, under the forms of glue, size, isinglass, and transparent jelly.

CAROLINE.

But these are of a very different nature; they cannot therefore be all pure gelatine.

MRS. B.

Not entirely, but very nearly so. Glue is extracted from the skin of animals. Size is obtained either from skin in its natural state, or from leather. Isinglass is gelatine procured from a particular species of fish; it is, you know, of this substance that the finest jelly is made, and this is done by merely dissolving the isinglass in boiling water, and allowing the solution to congeal.

EMILY.

The wine, lemon, and spices, are, I suppose, added only to flavour the jelly?

MRS. B.

Exactly so.

CAROLINE.

But jelly is often made of hartshorn shavings, and of calves' feet; do these substances contain gelatine?

MRS. B.

Yes. Gelatine may be obtained from almost any animal substance, as it enters more or less into the composition of all of them. The process for obtaining it is extremely simple, as it consists merely in boiling the substance that contains it with water. The gelatine dissolves in water, and may be attained of any degree of consistence or strength, by evaporating this solution. Bones in particular produce it very plentifully, as they consist of phosphat of lime combined or cemented by gelatine. Horns, which are a species of bone, will yield abundance of gelatine. The horns of the hart are reckoned to produce gelatine of the finest quality; they are reduced to the state of shavings in order that the jelly may be more easily extracted by the water. It is of hartshorn shavings that the jellies for invalids are usually made, as they are of very easy digestion.

CAROLINE.

It appears singular that hartshorn, which yields such a powerful ingredient as ammonia, should at the same time produce so mild and insipid a substance as jelly?

MRS. B.

And (what is more surprising) it is from the gelatine of bones that ammonia is produced. You must observe, however, that the processes by which these two substances are obtained from bones are very different. By the simple action of water and heat, the gelatine is separated; but in order to procure the ammonia, or what is commonly called hartshorn, the bones must be distilled, by which means the gelatine is decomposed, and hydrogen and nitrogen combined in the form of ammonia. So that the first operation is a mere separation of ingredients, whilst the second requires a chemical decomposition.

CAROLINE.

But when jelly is made from hartshorn shavings, what becomes of the phosphat of lime which constitutes the other part of bones?

MRS. B.

It is easily separated by straining. But the jelly is afterwards more perfectly purified, and rendered transparent, by adding white of egg, which being coagulated by heat, rises to the surface along with any impurities.

EMILY.

I wonder that bones are not used by the common people to make jelly; a great deal of wholesome nourishment, might, I should suppose, be procured from them, though the jelly would perhaps not be quite so good as if made from hartshorn shavings?

MRS. B.

There is a prejudice among the poor against a species of food that is usually thrown to the dogs; and as we cannot expect them to enter into chemical considerations, it is in some degree excusable. Besides, it requires a prodigious quantity of fuel to dissolve bones and obtain the gelatine from them.

The solution of bones in water is greatly promoted by an accumulation of heat. This may be effected by means of an extremely strong metallic vessel, called Papin's digester, in which the bones and water are enclosed, without any possibility of the steam making its escape. A heat can thus be applied much superior to that of boiling water; and bones, by this means, are completely reduced to a pulp. But the process still consumes too much fuel to be generally adopted among the lower classes.

CAROLINE.

And why should not a manufacture be established for grinding or macerating bones, or at least for reducing them to the state of shavings, when I suppose they would dissolve as readily as hartshorn shavings?

MRS. B.

They could not be collected clean for such a purpose, but they are not lost, as they are used for making hartshorn and sal ammoniac; and such is the superior science and industry of this country, that we now send sal ammoniac to the Levant, though it originally came to us from Egypt.

EMILY.

When jelly is made of isinglass, does it leave no sediment?

MRS. B.

No; nor does it so much require clarifying, as it consists almost entirely of pure gelantine, and any foreign matter that is mixed with it, is thrown off during the boiling in the form of scum. —These are processes which you may see performed in great perfection in the culinary laboratory, by that very able and most useful chemist the cook.

CAROLINE.

To what an immense variety of purposes chemistry is subservient!

EMILY.

It appears, in that respect, to have an advantage over most other arts and sciences; for these, very often, have a tendency to confine the imagination to their own particular object, whilst the pursuit of chemistry is so extensive and diversified, that it inspires a general curiosity, and a desire of enquiring into the nature of every object.

CAROLINE.

I suppose that soup is likewise composed of gelatine; for, when cold, it often assumes the consistence of jelly?

MRS. B.

Not entirely; for though soups generally contain a quantity of gelatine, the most essential ingredient is a mucous or extractive matter, a peculiar animal substance, very soluble in water, which has a strong taste, and is more nourishing than gelatine. The various kinds of portable soup consist of this extractive matter in a dry state, which, in order to be made into soup, requires only to be dissolved in water.

Gelatine, in its solid state, is a semiductile transparent substance, without either taste or smell. —When exposed to heat, in contact with air and water, it first swells, then fuses, and finally burns. You may have seen the first part of this operation performed in the carpenter's glue-pot.

CAROLINE.

But you said that gelatine had no smell, and glue has a very disagreeable one.

MRS. B.

Glue is not pure gelatine; as it is not designed for eating, it is prepared without attending to the state of the ingredients, which are more or less contaminated by particles that have become putrid.

Gelatine may be precipitated from its solution in water by alcohol. —We shall try this experiment with a glass of warm jelly. —You see that the gelatine subsides by the union of the alcohol and the water.

EMILY.

How is it, then, that jelly is flavoured with wine, without producing any precipitation?

MRS. B.

Because the alcohol contained in wine is already combined with water, and other ingredients, and is therefore not at liberty to act upon the jelly as when in its separate state. Gelatine is soluble both in acids and in alkalies; the former, you know, are frequently used to season jellies.

CAROLINE.

Among the combinations of gelatine we must not forget one which you formerly mentioned; that with tannin, to form leather.

MRS. B.

True; but you must observe that leather can be produced only by gelatine in a membranous state; for though pure gelatine and tannin will produce a substance chemically similar to leather, yet the texture of the skin is requisite to make it answer the useful purposes of that substance.

The next animal substance we are to examine is albumen; this, although constituting a part of most of the animal compounds, is frequently found insulated in the animal system; the white of egg, for instance, consists almost entirely of albumen; the substance that composes the nerves, the serum, or white part of the blood, and the curds of milk, are little else than albumen variously modified.

In its most simple state, albumen appears in the form of a transparent viscous fluid, possessed of no distinct taste or smell; it coagulates at the low temperature of 165 degrees, and, when once solidified, it will never return to its fluid state.

Sulphuric acid and alcohol are each of them capable of coagulating albumen in the same manner as heat, as I am going to show you.

EMILY.

Exactly so. —Pray, Mrs. B., what kind of action is there between albumen and silver? I have sometimes observed, that if the spoon with which I eat an egg happens to be wetted, it becomes tarnished.

MRS. B.

It is because the white of egg (and, indeed, albumen in general) contains a little sulphur, which, at the temperature of an egg just boiled, will decompose the drop of water that wets the spoon, and produce sulphurated hydrogen gas, which has the property of tarnishing silver.

We may now proceed to fibrine. This is an insipid and inodorous substance, having somewhat the appearance of fine white threads adhering together; it is the essential constituent of muscles or flesh, in which it is mixed with and softened by gelatine. It is insoluble both in water and alcohol, but sulphuric acid converts it into a substance very analogous to gelatine.

These are the essential and general ingredients of animal matter; but there are other substances, which, though not peculiar to the animal system, usually enter into its composition, such as oils, acids, salts, &c.

Animal oil is the chief constituent of fat; it is contained in abundance in the cream of milk, whence it is obtained in the form of butter.

EMILY.

Is animal oil the same in its composition as vegetable oils?

MRS. B.

Not the same, but very analogous. The chief difference is that animal oil contains nitrogen, a principle which seldom enters into the composition of vegetable oils, and never in so large a proportion.

There are a few animal acids, that is to say, acids peculiar to animal matter, from which they are almost exclusively obtained.

The animal acids have triple bases of hydrogen, carbon, and nitrogen. Some of them are found native in animal matter; others are produced during its decomposition.

Those that we find ready formed are:

The bombic acid, which is obtained from silk-worms.

The formic acid, from ants.

The lactic acid, from the whey of milk.

The sebacic, from oil or fat.

Those produced during the decomposition of animal substances by heat, are the prussic and zoonic acids. This last is produced by the roasting of meat, and gives it a brisk flavour.

CAROLINE.

The class of animal acids is not very extensive?

MRS. B.

No; nor are they, generally speaking, of great importance. The prussic acid is, I think, the only one sufficiently interesting to require any further comment. It can be formed by any artificial process, without the presence of any animal matter; and it may likewise be obtained from a variety of vegetables, particularly those of the narcotic kind, such as poppies, laurel, &c. But it is commonly obtained from blood, by strongly heating that substance with caustic potash; the alkali attracts the acid from the blood, and forms with it a prussiat of potash. From this state of combination the prussic acid can be obtained pure by means of other substances which have the power of separating it from the alkali.

EMILY.

But if this acid does not exist ready formed in blood, how can the alkali attract it from it?

MRS. B.

It is the triple basis only of this acid that exists in the blood; and this is developed and brought to the state of acid, during the combustion. The acid therefore is first formed, and it afterwards combines with the potash.

EMILY.

Now I comprehend it. But how can the prussic acid be artificially made?

MRS. B.

By passing ammoniacal gas over red-hot charcoal; and hence we learn that the constituents of this acid are hydrogen, nitrogen, and carbon. The two first are derived from the volatile alkali, the last from the combustion of the charcoal.

CAROLINE.

But this does not accord with the system of oxygen being the principle of acidity.

MRS. B.

The colouring matter of prussian blue is called an acid, because it unites with alkalies and metals, and not from any other characteristic properties of acids; perhaps the name is not strictly appropriate. But this circumstance, together with some others of the same kind, has induced several chemists to think that oxygen may not be the exclusive generator of acids. Sir H. Davy, I have already informed you, was led by his experiments on dry acids to suspect that water might be essential to acidity. And it is the opinion of some chemists that acidity may possibly depend rather on the arrangement than on the presence of any particular principles. But we have not yet done with the prussic acid. It has a strong affinity for metallic oxyds, and precipitates the solutions of iron in acids of a blue colour. This is the prussian blue, or prussiat of iron, so much used in the arts, and with which I think you must be acquainted.

EMILY.

Yes, I am; it is much used in painting, both in oil and in water colours; but it is not reckoned a permanent oil-colour.

MRS. B.

That defect arises, I believe, in general, from its being badly prepared, which is the case when the iron is not so fully oxydated as to form a red oxyd. For a solution of green oxyd of iron (in which the metal is more slightly oxydated), makes only a pale green, or even a white precipitate, with prussiat of potash; and this gradually changes to blue by being exposed to the air, as I can immediately show you.

CAROLINE.

It already begins to assume a pale blue colour. But how does the air produce this change?

MRS. B.

By oxydating the iron more perfectly. If we pour some nitrous acid on it, the prussian blue colour will be immediately produced, as the acid will yield its oxygen to the precipitate, and fully saturate it with this principle, as you shall see.

CAROLINE.

It is very curious to see a colour change so instantaneously.

MRS. B.

Hence you perceive that prussian blue cannot be a permanent colour, unless prepared with red oxyd of iron, since by exposure to the atmosphere it gradually darkens, and in a short time is no longer in harmony with the other colours of the painting.

CAROLINE.

But it can never become darker, by exposure to the atmosphere, than the true prussian blue, in which the oxyd is perfectly saturated?

MRS. B.

Certainly not. But in painting, the artist not reckoning upon partial alterations in his colours, gives his blue tints that particular shade which harmonises with the rest of the picture. If, afterwards, those tints become darker, the harmony of the colouring must necessarily be destroyed.

CAROLINE.

Pray, of what nature is the paint called carmine?

MRS. B.

It is an animal colour prepared from cochineal, an insect, the infusion of which produces a very beautiful red.

CAROLINE.

Whilst we are on the subject of colours, I should like to learn what ivory black is?

MRS. B.

It is a carbonaceous substance obtained by the combustion of ivory. A more common species of black is obtained from the burning of bone.

CAROLINE.

But during the combustion of ivory or bone, the carbon, I should have imagined, must be converted into carbonic acid gas, instead of this black substance?

MRS. B.

In this, as in most combustions, a considerable part of the carbon is simply volatilised by the heat, and again obtained concrete on cooling. This colour, therefore, may be called the soot produced by the burning of ivory or bone.



CONVERSATION XXIV.

ON THE ANIMAL ECONOMY.

MRS. B.

We have now acquired some idea of the various materials that compose the animal system; but if you are curious to know in what manner these substances are formed by the animal organs, from vegetable, as well as from animal substances, it will be necessary to have some previous knowledge of the nature and functions of these organs, without which it is impossible to form any distinct idea of the process of animalisation and nutrition.

CAROLINE.

I do not exactly understand the meaning of the word animalisation?

MRS. B.

Animalisation is the process by which the food is assimilated, that is to say, converted into animal matter; and nutrition is that by which the food thus assimilated is rendered subservient to the purposes of nourishing and maintaining the animal system.

EMILY.

This, I am sure, must be the most interesting of all the branches of chemistry!

CAROLINE.

So I think; particularly as I expect that we shall hear something of the nature of respiration, and of the circulation of the blood?

MRS. B.

These functions undoubtedly occupy a most important place in the history of the animal economy. —But I must previously give you a very short account of the principal organs by which the various operations of the animal system are performed. These are:

The Bones; Muscles, Blood vessels, Lymphatic vessels, Glands, and Nerves.

The bones are the most solid part of the animal frame, and in a great measure determine its form and dimensions. You recollect, I suppose, what are the ingredients which enter into their composition?

CAROLINE.

Yes; phosphat of lime, cemented by gelatine.

MRS. B.

During the earliest period of animal life, they consist almost entirely of gelatinous membrane having the form of the bones, but of a loose spongy texture, the cells or cavities of which are destined to be filled with phosphat of lime; it is the gradual acquisition of this salt which gives to the bones their subsequent hardness and durability. Infants first receive it from their mother's milk, and afterwards derive it from all animal and from most vegetable food, especially farinaceous substances, such as wheat-flour, which contain it in sensible quantities. A portion of the phosphat, after the bones of the infant have been sufficiently expanded and solidified, is deposited in the teeth, which consist at first only of a gelatinous membrane or case, fitted for the reception of this salt; and which, after acquiring hardness within the gum, gradually protrude from it.

CAROLINE.

How very curious this is; and how ingeniously nature has first provided for the solidification of such bones as are immediately wanted, and afterwards for the formation of the teeth, which would not only be useless, but detrimental in infancy!

MRS. B.

In quadrupeds the phosphat of lime is deposited likewise in their horns, and in the hair or wool with which they are generally clothed.

In birds it serves also to harden the beaks and the quills of their feathers.

When animals are arrived at a state of maturity, and their bones have acquired a sufficient degree of solidity, the phosphat of lime which is taken with the food is seldom assimilated, excepting when the female nourishes her young; it is then all secreted into the milk, as a provision for the tender bones of the nursling.

EMILY.

So that whatever becomes superfluous to one being, is immediately wanted by another; and the child acquires strength precisely by the species of nourishment which is no longer necessary to the mother. Nature is, indeed, an admirable economist!

CAROLINE.

Pray, Mrs. B., does not the disease in the bones of children, called the rickets, proceed from a deficiency of phosphat of lime?

MRS. B.

I have heard that this disease may arise from two causes; it is sometimes occasioned by the growth of the muscles being too rapid in proportion to that of the bones. In this case the weight of the flesh is greater than the bones can support, and presses upon them so as to produce a swelling of the joints, which is the great indication of the rickets. The other cause of this disorder is supposed to be an imperfect digestion and assimilation of the food, attended with an excess of acid, which counteracts the formation of phosphat of lime. In both instances, therefore, care should be taken to alter the child's diet, not merely by increasing the quantity of aliment containing phosphat of lime, but also by avoiding all food that is apt to turn acid on the stomach, and to produce indigestion. But the best preservative against complaints of this kind is, no doubt, good nursing: when a child has plenty of air and exercise, the digestion and assimilation will be properly performed, no acid will be produced to interrupt these functions, and the muscles and bones will grow together in just proportions.

CAROLINE.

I have often heard the rickets attributed to bad nursing, but I never could have guessed what connection there was between exercise and the formation of the bones.

MRS. B.

Exercise is generally beneficial to all the animal functions. If man is destined to labour for his subsistence, the bread which he earns is scarcely more essential to his health and preservation than the exertions by which he obtains it. Those whom the gifts of fortune have placed above the necessity of bodily labour are compelled to take exercise in some mode or other, and when they cannot convert it into an amusement, they must submit to it as a task, or their health will soon experience the effects of their indolence.

EMILY.

That will never be my case: for exercise, unless it becomes fatigue, always gives me pleasure; and, so far from being a task, is to me a source of daily enjoyment. I often think what a blessing it is, that exercise, which is so conducive to health, should be so delightful; whilst fatigue, which is rather hurtful, instead of pleasure, occasions painful sensations. So that fatigue, no doubt, was intended to moderate our bodily exertions, as satiety puts a limit to our appetites.

MRS. B.

Certainly. —But let us not deviate too far from our subject. —The bones are connected together by ligaments, which consist of a white thick flexible substance, adhering to their extremities, so far as to secure the joints firmly, though without impeding their motion. And the joints are moreover covered by a solid, smooth, elastic, white substance, called cartilage, the use of which is to allow, by its smoothness and elasticity, the bones to slide easily over one another, so that the joints may perform their office without difficulty or detriment.

Over the bones the muscles are placed; they consist of bundles of fibres which terminate in a kind of string, or ligament, by which they are fastened to the bones. The muscles are the organs of motion; by their power of dilatation and contraction they put into action the bones, which act as levers, in all the motions of the body, and form the solid support of its various parts. The muscles are of various degrees of strength or consistence in different species of animals. The mammiferous tribe, or those that suckle their young, seem in this respect to occupy an intermediate place between birds and cold-blooded animals, such as reptiles and fishes.

EMILY.

The different degrees of firmness and solidity in the muscles of these several species of animals proceed, I imagine, from the different nature of the food on which they subsist?

MRS. B.

No; that is not supposed to be the case: for the human species, who are of the mammiferous tribe, live on more substantial food than birds, and yet the latter exceed them in muscular strength. We shall hereafter attempt to account for this difference; but let us now proceed in the examination of the animal functions.

The next class of organs is that of the vessels of the body, the office of which is to convey the various fluids throughout the frame. These vessels are innumerable. The most considerable of them are those through which the blood circulates, which are of two kinds: the arteries, which convey it from the heart to the extremities of the body, and the veins, which bring it back into the heart.

Besides these, there are a numerous set of small transparent vessels, destined to absorb and convey different fluids into the blood; they are generally called the absorbent or lymphatic vessels: but it is to a portion of them only that the function of conveying into the blood the fluid called lymph is assigned.

EMILY.

Pray what is the nature of that fluid?

MRS. B.

The nature and use of the lymph have, I believe, never been perfectly ascertained; but it is supposed to consist of matter that has been previously animalised, and which, after answering the purpose for which it was intended, must, in regular rotation, make way for the fresh supplies produced by nourishment. The lymphatic vessels pump up this fluid from every part of the system, and convey it into the veins to be mixed with the blood which runs through them, and which is commonly called venous blood.

CAROLINE.

But does it not again enter into the animal system through that channel?

MRS. B.

Not entirely; for the venous blood does not return into the circulation until it has undergone a peculiar change, in which it throws off whatever is become useless.

Another set of absorbent vessels pump up the chyle from the stomach and intestines, and convey it, after many circumvolutions, into the great vein near the heart.

EMILY.

Pray what is chyle?

MRS. B.

It is the substance into which food is converted by digestion.

CAROLINE.

One set of the absorbent vessels, then, is employed in bringing away the old materials that are no longer fit for use; whilst the other set is busy in conveying into the blood the new materials that are to replace them.

EMILY.

What a great variety of ingredients must enter into the composition of the blood?

MRS. B.

You must observe that there is also a great variety of substances to be secreted from it. We may compare the blood to a general receptacle or storehouse for all kinds of commodities, which are afterwards fashioned, arranged, and disposed of as circumstances require.

There is another set of absorbent vessels in females which is destined to secrete milk for the nourishment of the young.

EMILY.

Pray is not milk very analogous in its composition to blood; for, since the nursling derives its nourishment from that source only, it must contain every principle which the animal system requires?

MRS. B.

Very true. Milk is found, by its analysis, to contain the principal materials of animal matter, albumen, oil, and phosphat of lime; so that the suckling has but little trouble to digest and assimilate this nourishment. But we shall examine the composition of milk more fully afterwards.

In many parts of the body numbers of small vessels are collected together in little bundles called glands, from a Latin word meaning acorn, on account of the resemblance which some of them bear in shape to that fruit. The function of the glands is to secrete, or separate certain matters from the blood.

The secretions are not only mechanical, but chemical separations from the blood; for the substances thus formed, though contained in the blood, are not ready combined in that fluid. The secretions are of two kinds, those which form peculiar animal fluids, as bile, tears, saliva, &c.; and those which produce the general materials of the animal system, for the purpose of recruiting and nourishing the several organs of the body; such as albumen, gelatine, and fibrine; the latter may be distinguished by the name of nutritive secretions.

CAROLINE.

I am quite astonished to hear that all the secretions should be derived from the blood.

EMILY.

I thought that the bile was produced by the liver?

MRS. B.

So it is; but the liver is nothing more than a very large gland, which secretes the bile from the blood.

The last of the animal organs which we have mentioned are the nerves; these are the vehicles of sensation, every other part of the body being, of itself, totally insensible.

CAROLINE.

They must then be spread through every part of the frame, for we are every where susceptible of feeling.

EMILY.

Excepting the nails and the hair.

MRS. B.

And those are almost the only parts in which nerves cannot be discovered. The common source of all the nerves is the brain; thence they descend, some of them through different holes of the skull, but the greatest part through the back bone, and extend themselves by innumerable ramifications throughout the whole body. They spread themselves over the muscles, penetrate the glands, wind round the vascular system, and even pierce into the interior of the bones. It is most probably through them that the communication is carried on between the mind and the other parts of the body; but in what manner they are acted on by the mind, and made to re-act on the body, is still a profound secret. Many hypotheses have been formed on this very obscure subject, but they are all equally improbable, and it would be useless for us to waste our time in conjectures on an enquiry, which, in all probability, is beyond the reach of human capacity.

CAROLINE.

But you have not mentioned those particular nerves that form the senses of hearing, seeing, smelling, and tasting?

MRS. B.

They are considered as being of the same nature as those which are dispersed over every part of the body, and constitute the general sense of feeling. The different sensations which they produce arise from their peculiar situation and connection with the several organs of taste, smell, and hearing.

EMILY.

But these senses appear totally different from that of feeling?

MRS. B.

They are all of them sensations, but variously modified according to the nature of the different organs in which the nerves are situated. For, as we have formerly observed, it is by contact only that the nerves are affected. Thus odoriferous particles must strike upon the nerves of the nose, in order to excite the sense of smelling; in the same manner that taste is produced by the particular substance coming in contact with the nerves of the palate. It is thus also that the sensation of sound is produced by the concussion of the air striking against the auditory nerve; and sight is the effect of the light falling upon the optic nerve. These various senses, therefore, are affected only by the actual contact of particles of matter, in the same manner as that of feeling.

The different organs of the animal body, though easily separated and perfectly distinct, are loosely connected together by a kind of spongy substance, in texture somewhat resembling net-work, called the cellular membrane; and the whole is covered by the skin.

The skin, as well as the bark of vegetables, is formed of three coats. The external one is called the cuticle or epidermis; the second, which is called the mucous membrane, is of a thin soft texture, and consists of a mucous substance, which in negroes is black, and is the cause of their skin appearing of that colour.

CAROLINE.

Is then the external skin of negroes white like ours?

MRS. B.

Yes; but as the cuticle is transparent, as well as porous, the blackness of the mucous membrane is visible through it. The extremities of the nerves are spread over this skin, so that the sensation of feeling is transmitted through the cuticle. The internal covering of the muscles, which is properly the skin, is the thickest, the toughest, and most resisting of the whole; it is this membrane which is so essential in the arts, by forming leather when combined with tannin.

The skin which covers the animal body, as well as those membranes that form the coats of the vessels, consists almost exclusively of gelatine; and is capable of being converted into glue, size, or jelly.

The cavities between the muscles and the skin are usually filled with fat, which lodges in the cells of the membranous net before mentioned, and gives to the external form (especially in the human figure) that roundness, smoothness, and softness, so essential to beauty.

EMILY.

And the skin itself is, I think, a very ornamental part of the human frame, both from the fineness of its texture, and the variety and delicacy of its tints.

MRS. B.

This variety and harmonious graduation of colours, proceed, not so much from the skin itself, as from the internal organs which transmit their several colours through it, these being only softened and blended by the colour of the skin, which is uniformly of a yellowish white.

Thus modified, the darkness of the veins appears of a pale blue colour, and the floridness of the arteries is changed to a delicate pink. In the most transparent parts, the skin exhibits the bloom of the rose, whilst where it is more opake its own colour predominates; and at the joints, where the bones are most prominent, their whiteness is often discernible. In a word, every part of the human frame seems to contribute to its external grace; and this not merely by producing a pleasing variety of tints, but by a peculiar kind of beauty which belongs to each individual part. Thus it is to the solidity and arrangement of the bones that the human figure owes the grandeur of its stature, and its firm and dignified deportment. The muscles delineate the form, and stamp it with energy and grace; and the soft substance which is spread over them smooths their ruggedness, and gives to the contours the gentle undulations of the line of beauty. Every organ of sense is a peculiar and separate ornament; and the skin, which polishes the surface, and gives it that charm of colouring so inimitable by art, finally conspires to render the whole the fairest work of the creation.

But now that we have seen in what manner the animal frame is formed, let us observe how it provides for its support, and how the several organs, which form so complete a whole, are nourished and maintained.

This will lead us to a more particular explanation of the internal organs: here we shall not meet with so much apparent beauty, because these parts were not intended by nature to be exhibited to view; but the beauty of design, in the internal organisation of the animal frame, is, if possible, still more remarkable than that of the external parts.

We shall defer this subject till our next interview.



CONVERSATION XXV.

ON ANIMALISATION, NUTRITION, AND RESPIRATION.

MRS. B.

We have now learnt of what materials the animal system is composed, and have formed some idea of the nature of its organisation. In order to complete the subject, it remains for us to examine in what manner it is nourished and supported.

Vegetables, we have observed, obtain their nourishment from various substances, either in their elementary state, or in a very simple state of combination; as carbon, water, and salts, which they pump up from the soil; and carbonic acid and oxygen, which they absorb from the atmosphere.

Animals, on the contrary, feed on substances of the most complicated kind; for they derive their sustenance, some from the animal creation, others from the vegetable kingdom, and some from both.

CAROLINE.

And there is one species of animals, which, not satisfied with enjoying either kind of food in its simple state, has invented the art of combining them together in a thousand ways, and of rendering even the mineral kingdom subservient to its refinements.

EMILY.

Nor is this all; for our delicacies are collected from the various climates of the earth, so that the four quarters of the globe are often obliged to contribute to the preparation of our simplest dishes.

CAROLINE.

But the very complicated substances which constitute the nourishment of animals, do not, I suppose, enter into their system in their actual state of combination?

MRS. B.

So far from it, that they not only undergo a new arrangement of their parts, but a selection is made of such as are most proper for the nourishment of the body, and those only enter into the system, and are animalised.

EMILY.

And by what organs is this process performed?

MRS. B.

Chiefly by the stomach, which is the organ of digestion, and the prime regulator of the animal frame.

Digestion is the first step towards nutrition. It consists in reducing into one homogeneous mass the various substances that are taken as nourishment; it is performed by first chewing and mixing the solid aliment with the saliva, which reduces it to a soft mass, in which state it is conveyed into the stomach, where it is more completely dissolved by the gastric juice.

This fluid (which is secreted into the stomach by appropriate glands) is so powerful a solvent that scarcely any substances will resist its action.

EMILY.

The coats of the stomach, however, cannot be attacked by it, otherwise we should be in danger of having them destroyed when the stomach was empty.

MRS. B.

They are probably not subject to its action; as long, at least, as life continues. But it appears, that when the gastric juice has no foreign substance to act upon, it is capable of occasioning a degree of irritation in the coats of the stomach, which produces the sensation of hunger. The gastric juice, together with the heat and muscular action of the stomach, converts the aliment into an uniform pulpy mass called chyme. This passes into the intestines, where it meets with the bile and some other fluids, by the agency of which, and by the operation of other causes hitherto unknown, the chyme is changed into chyle, a much thinner substance, somewhat resembling milk, which is pumped by immense numbers of small absorbent vessels spread over the internal surface of the intestines. These, after many circumvolutions, gradually meet and unite into large branches, till they at length collect the chyle into one vessel, which pours its contents into the great vein near the heart, by which means the food, thus prepared, enters into the circulation.

CAROLINE.

But I do not yet clearly understand how the blood, thus formed, nourishes the body and supplies all the secretions?

MRS. B.

Before this can be explained to you, you must first allow me to complete the formation of the blood. The chyle may, indeed, be considered as forming the chief ingredient of blood; but this fluid is not perfect until it has passed through the lungs, and undergone (together with the blood that has already circulated) certain necessary changes that are effected by RESPIRATION.

CAROLINE.

I am very glad that you are going to explain the nature of respiration: I have often longed to understand it, for though we talk incessantly of breathing, I never knew precisely what purpose it answered.

MRS. B.

It is indeed one of the most interesting processes imaginable; but, in order to understand this function well, it will be necessary to enter into some previous explanations. Tell me, Emily, —what do you understand by respiration?

EMILY.

Respiration, I conceive, consists simply in alternately inspiring air into the lungs, and expiring it from them.

MRS. B.

Your answer will do very well as a general definition. But, in order to form a tolerably clear notion of the various phenomena of respiration, there are many circumstances to be taken into consideration.

In the first place, there are two things to be distinguished in respiration, the mechanical and the chemical part of the process.

The mechanism of breathing depends on the alternate expansions and contractions of the chest, in which the lungs are contained. When the chest dilates, the cavity is enlarged, and the air rushes in at the mouth, to fill up the vacuum formed by this dilatation; when it contracts, the cavity is diminished, and the air forced out again.

CAROLINE.

I thought that it was the lungs that contracted and expanded in breathing?

MRS. B.

They do likewise; but their action is only the consequence of that of the chest. The lungs, together with the heart and largest blood vessels, in a manner fill up the cavity of the chest; they could not, therefore, dilate if the chest did not previously expand; and, on the other hand, when the chest contracts, it compresses the lungs and forces the air out of them.

CAROLINE.

The lungs, then, are like bellows, and the chest is the power that works them.

MRS. B.

Precisely so. Here is a curious little figure (PLATE XV. Fig. 5.), that will assist me in explaining the mechanism of breathing.

CAROLINE.

What a droll figure! a little head fixed upon a glass bell, with a bladder tied over the bottom of it!

MRS. B.

You must observe that there is another bladder within the glass, the neck of which communicates with the mouth of the figure—this represents the lungs contained within the chest; the other bladder, which you see is tied loose, represents a muscular membrane, called the diaphragm, which separates the chest from the lower part of the body. By the chest, therefore, I mean that large cavity in the upper part of the body contained within the ribs, the neck, and the diaphragm; this membrane is muscular, and capable of contraction and dilatation. The contraction may be imitated by drawing the bladder tight over the bottom of the receiver, when the air in the bladder, which represents the lungs, will be forced out through the mouth of the figure—

EMILY.

See, Caroline, how it blows the flame of the candle in breathing!

MRS. B.

By letting the bladder loose again, we imitate the dilatation of the diaphragm, and the cavity of the chest being enlarged, the lungs expand, and the air rushes in to fill them.

EMILY.

This figure, I think, gives a very clear idea of the process of breathing.

MRS. B.

It illustrates tolerably well the action of the lungs and diaphragm; but those are not the only powers that are concerned in enlarging or diminishing the cavity of the chest; the ribs are also possessed of a muscular motion for the same purpose; they are alternately drawn in, edgeways, to assist the contraction, and stretched out, like the hoops of a barrel, to contribute to the dilatation of the chest.

EMILY.

I always supposed that the elevation and depression of the ribs were the consequence, not the cause of breathing.

MRS. B.

It is exactly the reverse. The muscular action of the diaphragm, together with that of the ribs, are the causes of the contraction and expansion of the chest; and the air rushing into, and being expelled from the lungs, are only consequences of those actions.

CAROLINE.

I confess that I thought the act of breathing began by opening the mouth for the air to rush in, and that it was the air alone, which, by alternately rushing in and out, occasioned the dilatations and contractions of the lungs and chest.

MRS. B.

Try the experiment of merely opening your mouth; the air will not rush in, till by an interior muscular action you produce a vacuum—yes, just so, your diaphragm is now dilated, and the ribs expanded. But you will not be able to keep them long in that state. Your lungs and chest are already resuming their former state, and expelling the air with which they had just been filled. This mechanism goes on more or less rapidly, but, in general, a person at rest and in health will breathe between fifteen and twenty-five times in a minute.

We may now proceed to the chemical effects of respiration; but, for this purpose, it is necessary that you should previously have some notion of the circulation of the blood. Tell me, Caroline, what do you understand by the circulation of the blood?

CAROLINE.

I am delighted that you come to that subject, for it is one that has long excited my curiosity. But I cannot conceive how it is connected with respiration. The idea I have of the circulation is, that the blood runs from the heart through the veins all over the body, and back again to the heart.

MRS. B.

I could hardly have expected a better definition from you; it is, however, not quite correct, for you do not distinguish the arteries from the veins, which, as we have already observed, are two distinct sets of vessels, each having its own peculiar functions. The arteries convey the blood from the heart to the extremities of the body; and the veins bring it back into the heart.

This sketch will give you an idea of the manner in which some of the principal veins and arteries of the human body branch out of the heart, which may be considered as a common centre to both sets of vessels. The heart is a kind of strong elastic bag, or muscular cavity, which possesses a power of dilating and contracting itself, for the purposes of alternately receiving and expelling the blood, in order to carry on the process of circulation.

EMILY.

Why are the arteries in this drawing painted red, and the veins purple?

MRS. B.

It is to point out the difference of the colour of the blood in these two sets of vessels.

CAROLINE.

But if it is the same blood that flows from the arteries into the veins, how can its colour be changed?

MRS. B.

This change arises from various circumstances. In the first place, during its passage through the arteries, the blood undergoes a considerable alteration, some of its constituent parts being gradually separated from it for the purpose of nourishing the body, and of supplying the various secretions. The consequence of this is, that the florid arterial colour of the blood changes by degrees to a deep purple, which is its constant colour in the veins. On the other hand, the blood is recruited during its return through the veins by the fresh chyle, or imperfect blood, which has been produced by food; and it receives also lymph from the absorbent vessels, as we have before mentioned. In consequence of these several changes, the blood returns to the heart in a state very different from that in which it left it. It is loaded with a greater proportion of hydrogen and carbon, and is no longer fit for the nourishment of the body, or other purposes of circulation.

EMILY.

And in this state does it mix in the heart with the pure florid blood that runs into the arteries?

MRS. B.

No. The heart is divided into two cavities or compartitions, called the right and left ventricles. The left ventricle is the receptacle for the pure arterial blood previous to its circulation; whilst the venous, or impure blood, which returns to the heart after having circulated, is received into the right ventricle, previous to its purification, which I shall presently explain.

CAROLINE.

For my part, I always thought that the same blood circulated again and again through the body, without undergoing any change.

MRS. B.

Yet you must have supposed that the blood circulated for some purpose?

CAROLINE.

I knew that it was indispensable to life; but had no idea of its real functions.

MRS. B.

But now that you understand that the blood conveys nourishment to every part of the body, and supplies the various secretions, you must be sensible that it cannot constantly answer these objects without being proportionally renovated and purified.

CAROLINE.

But does not the chyle answer this purpose?

MRS. B.

Only in part. It renovates the nutritive principles of the blood, but does not relieve it from the superabundance of water and carbon with which it is encumbered.

EMILY.

How, then, is this effected?

MRS. B.

By RESPIRATION. This is one of the grand mysteries which modern chemistry has disclosed. When the venous blood enters the right ventricle of the heart, it contracts by its muscular power, and throws the blood through a large vessel into the lungs, which are contiguous, and through which it circulates by millions of small ramifications. Here it comes in contact with the air which we breathe. The action of the air on the blood in the lungs is, indeed, concealed, from our immediate observation; but we are able to form a tolerably accurate judgment of it from the changes which it effects not only in the blood, but also on the air expired.

The air, after passing through the lungs, is found to contain all the nitrogen inspired, but to have lost part of its oxygen, and to have acquired a portion of watery vapour and of carbonic acid gas. Hence it is inferred, that when the air comes in contact with the venous blood in the lungs, the oxygen attracts from it the superabundant quantity of carbon with which it has impregnated itself during the circulation, and converts it into carbonic acid. This gaseous acid, together with the redundant moisture from the lungs*, being then expired, the blood is restored to its former purity, that is, to the state of arterial blood, and is thus again enabled to perform its various functions.

[Footnote *: The quantity of moisture discharged by the lungs in 24 hours, may be computed at eight or nine ounces.]

CAROLINE.

This is truly wonderful! Of all that we have yet learned, I do not recollect any thing that has appeared to me so curious and interesting. I almost believe that I should like to study anatomy now, though I have hitherto had so disgusting an idea of it. Pray, to whom are we indebted for these beautiful discoveries?

MRS. B.

Priestley and Crawford, in this country, and Lavoisier, in France, are the principal inventors of the theory of respiration. Of late years the subject has been farther illustrated and simplified by the accurate experiments of Messrs. Allen and Pepys. But the still more important and more admirable discovery of the circulation of the blood was made long before by our immortal countryman Harvey.

EMILY.

Indeed I never heard any thing that delighted me so much as this theory of respiration. But I hope, Mrs. B., that you will enter a little more into particulars before you dismiss so interesting a subject. We left the blood in the lungs to undergo the salutary change: but how does it thence spread to all the parts of the body?

MRS. B.

After circulating through the lungs, the blood is collected into four large vessels, by which it is conveyed into the left ventricle of the heart, whence it is propelled to all the different parts of the body by a large artery, which gradually ramifies into millions of small arteries through the whole frame. From the extremities of these little ramifications the blood is transmitted to the veins, which bring it back to the heart and lungs, to go round again and again in the manner we have just described. You see, therefore, that the blood actually undergoes two circulations; the one, through the lungs, by which it is converted into pure arterial blood; the other, or general circulation, by which nourishment is conveyed to every part of the body; and these are both equally indispensable to the support of animal life.

EMILY.

But whence proceeds the carbon with which the blood is impregnated when it comes into the lungs?

MRS. B.

Carbon exists in a greater proportion in blood than in organised animal matter. The blood, therefore, after supplying its various secretions, becomes loaded with an excess of carbon, which is carried off by respiration; and the formation of new chyle from the food affords a constant supply of carbonaceous matter.

CAROLINE.

I wonder what quantity of carbon may be expelled from the blood by respiration in the course of 24 hours?

MRS. B.

It appears by the experiments of Messrs. Allen and Pepys that about 40,000 cubic inches of carbonic acid gas are emitted from the lungs of a healthy person, daily; which is equivalent to eleven ounces of solid carbon every 24 hours.

EMILY.

What an immense quantity! And pray how much of carbonic acid gas do we expel from our lungs at each expiration?

MRS. B.

The quantity of air which we take into our lungs at each inspiration, is about 40 cubic inches, which contain a little less than 10 cubic inches of oxygen; and of those 10 inches, one-eighth is converted into carbonic acid gas on passing once through the lungs*, a change which is sufficient to prevent air which has only been breathed once from suffering a taper to burn in it.

[Footnote *: The bulk of carbonic acid gas formed by respiration, is exactly the same as that of the oxygen gas which disappears.]

CAROLINE.

Pray, how does the air come in contact with the blood in the lungs?

MRS. B.

I cannot answer this question without entering into an explanation of the nature and structure of the lungs. You recollect that the venous blood, on being expelled from the right ventricle, enters the lungs to go through what we may call the lesser circulation; the large trunk or vessel that conveys it branches out, at its entrance into the lungs, into an infinite number of very fine ramifications. The windpipe, which conveys the air from the mouth into the lungs, likewise spreads out into a corresponding number of air vessels, which follow the same course as the blood vessels, forming millions of very minute air-cells. These two sets of vessels are so interwoven as to form a sort of net-work, connected into a kind of spongy mass, in which every particle of blood must necessarily come in contact with a particle of air.

CAROLINE.

But since the blood and the air are contained in different vessels, how can they come into contact?

MRS. B.

They act on each other through the membrane which forms the coats of these vessels; for although this membrane prevents the blood and the air from mixing together in the lungs, yet it is no impediment to their chemical action on each other.

EMILY.

Are the lungs composed entirely of blood vessels and air vessels?

MRS. B.

I believe they are, with the addition only of nerves and of a small quantity of the cellular substance before mentioned, which connects the whole into an uniform mass.

EMILY.

Pray, why are the lungs always spoken of in the plural number? Are there more than one?

MRS. B.

Yes; for though they form but one organ, they really consist of two compartments called lobes, which are enclosed in separate membranes or bags, each occupying one side of the chest, and being in close contact with each other, but without communicating together. This is a beautiful provision of nature, in consequence of which, if one of the lobes be wounded, the other performs the whole process of respiration till the first is healed.

The blood, thus completed, by the process of respiration, forms the most complex of all animal compounds, since it contains not only the numerous materials necessary to form the various secretions, as saliva, tears, &c. but likewise all those that are required to nourish the several parts of the body, as the muscles, bones, nerves, glands, &c.

EMILY.

There seems to be a singular analogy between the blood of animals and the sap of vegetables; for each of these fluids contains the several materials destined for the nutrition of the numerous class of bodies to which they respectively belong.

MRS. B.

Nor is the production of these fluids in the animal and vegetable systems entirely different; for the absorbent vessels, which pump up the chyle from the stomach and intestines, may be compared to the absorbents of the roots of plants, which suck up the nourishment from the soil. And the analogy between the sap and the blood may be still further traced, if we follow the latter in the course of its circulation; for, in the living animal, we find every where organs which are possessed of a power to secrete from the blood and appropriate to themselves the ingredients requisite for their support.

CAROLINE.

But whence do these organs derive their respective powers?

MRS. B.

From a peculiar organisation, the secret of which no one has yet been able to unfold. But it must be ultimately by means of the vital principle that both their mechanical and chemical powers are brought into action.

I cannot dismiss the subject of circulation without mentioning perspiration, a secretion which is immediately connected with it, and acts a most important part in the animal economy.

CAROLINE.

Is not this secretion likewise made by appropriate glands?

MRS. B.

No; it is performed by the extremities of the arteries, which penetrate through the skin and terminate under the cuticle, through the pores of which the perspiration issues. When this fluid is not secreted in excess, it is insensible, because it is dissolved by the air as it exudes from the pores; but when it is secreted faster than it can be dissolved, it becomes sensible, as it assumes its liquid state.

EMILY.

This secretion bears a striking resemblance to the transpiration of the sap of plants. They both consist of the most fluid part, and both exude from the surface by the extremities of the vessels through which they circulate.

MRS. B.

And the analogy does not stop there; for, since it has been ascertained that the sap returns into the roots of the plants, the resemblance between the animal and vegetable circulation is become still more obvious. The latter, however, is far from being complete, since, as we observed before, it consists only in a rising and descending of the sap, whilst in animals the blood actually circulates through every part of the system.

We have now, I think, traced the process of nutrition, from the introduction of the food into the stomach to its finally becoming a constituent part of the animal frame. This will, therefore, be a fit period to conclude our present conversation. What further remarks we have to make on the animal economy shall be reserved for our next interview.



CONVERSATION XXVI.

ON ANIMAL HEAT; AND ON VARIOUS ANIMAL PRODUCTS.

EMILY.

Since our last interview, I have been thinking much of the theory of respiration; and I cannot help being struck with the resemblance which it appears to bear to the process of combustion. For in respiration, as in most cases of combustion, the air suffers a change, and a portion of its oxygen combines with carbon, producing carbonic acid gas.

MRS. B.

I am much pleased that this idea has occurred to you: these two processes appear so very analogous, that it has been supposed that a kind of combustion actually takes place in the lungs; not of the blood, but of the superfluous carbon which the oxygen attracts from it.

CAROLINE.

A combustion in our lungs! that is a curious idea indeed! But, Mrs. B., how can you call the action of the air on the blood in the lungs combustion, when neither light nor heat are produced by it?

EMILY.

I was going to make the same objection. —Yet I do not conceive how the oxygen can combine with the carbon, and produce carbonic acid, without disengaging heat?

MRS. B.

The fact is, that heat is disengaged.* Whether any light be evolved, I cannot pretend to determine; but that heat is produced in considerable and very sensible quantities is certain, and this is the principal, if not the only source of ANIMAL HEAT.

[Footnote *: It has been calculated that the heat produced by respiration in 12 hours, in the lungs of a healthy person, is such as would melt about 100 pounds of ice.]

EMILY.

How wonderful! that the very process which purifies and elaborates the blood, should afford an inexhaustible supply of internal heat?

MRS. B.

This is the theory of animal heat in its original simplicity, such nearly as it was first proposed by Black and Lavoisier. It was equally clear and ingenious; and was at first generally adopted. But it was objected, on second consideration, that if the whole of the animal heat was evolved in the lungs, it would necessarily be much less in the extremities of the body than immediately at its source; which is not found to be the case. This objection, however, which was by no means frivolous, is now satisfactorily removed by the following consideration:— Venous blood has been found by experiment to have less capacity for heat than arterial blood; whence it follows that the blood, in gradually passing from the arterial to the venous state, during the circulation, parts with a portion of caloric, by means of which heat is diffused through every part of the body.

EMILY.

More and more admirable!

CAROLINE.

The cause of animal heat was always a perfect mystery to me, and I am delighted with its explanation. —But pray, Mrs. B., can you tell me what is the reason of the increase of heat that takes place in a fever?

EMILY.

Is it not because we then breathe quicker, and therefore more heat is disengaged in the system?

MRS. B.

That may be one reason: but I should think that the principal cause of the heat experienced in fevers, is, that there is no vent for the caloric which is generated in the body. One of the most considerable secretions is the insensible perspiration; this is constantly carrying off caloric in a latent state; but during the hot stage of a fever, the pores are so contracted, that all perspiration ceases, and the accumulation of caloric in the body occasions those burning sensations which are so painful.

EMILY.

This is, no doubt, the reason why the perspiration that often succeeds the hot stage of a fever affords so much relief. If I had known this theory of animal heat when I had a fever last summer, I think I should have found some amusement in watching the chemical processes that were going on within me.

CAROLINE.

But exercise likewise produces animal heat, and that must be quite in a different manner.

MRS. B.

Not so much so as you think; for the more exercise you take, the more the body is stimulated, and requires recruiting. For this purpose the circulation of the blood is quickened, the breath proportionably accelerated, and consequently a greater quantity of caloric evolved.

CAROLINE.

True; after running very fast, I gasp for breath, my respiration is quick and hard, and it is just then that I begin to feel hot.

EMILY.

It would seem, then, that violent exercise should produce fever.

MRS. B.

Not if the person is in a good state of health; for the additional caloric is then carried off by the perspiration which succeeds.

EMILY.

What admirable resources nature has provided for us! By the production of animal heat she has enabled us to keep up the temperature of our bodies above that of inanimate objects; and whenever this source becomes too abundant, the excess is carried off by perspiration.

MRS. B.

It is by the same law of nature that we are enabled, in all climates, and in all seasons, to preserve our bodies of an equal temperature, or at least very nearly so.

CAROLINE.

You cannot mean to say that our bodies are of the same temperature in summer, and in winter, in England, and in the West-Indies.

MRS. B.

Yes, I do; at least if you speak of the temperature of the blood, and the internal parts of the body; for those parts that are immediately in contact with the atmosphere, such as the hands and face, will occasionally get warmer, or colder, than the internal or more sheltered parts. But if you put the bulb of a thermometer in your mouth, which is the best way of ascertaining the real temperature of your body, you will scarcely perceive any difference in its indication, whatever may be the difference of temperature of the atmosphere.

CAROLINE.

And when I feel overcome by heat, I am really not hotter than when I am shivering with cold?

MRS. B.

When a person in health feels very hot, whether from internal heat, from violent exercise, or from the temperature of the atmosphere, his body is certainly a little warmer than when he feels very cold; but this difference is much smaller than our sensations would make us believe; and the natural standard is soon restored by rest and by perspiration. It is chiefly the external parts that are warmer, and I am sure that you will be surprised to hear that the internal temperature of the body scarcely ever descends below ninety-five or ninety-six degrees, and seldom attains one hundred and four or one hundred and five degrees, even in the most violent fevers.

EMILY.

The greater quantity of caloric, therefore, that we receive from the atmosphere in summer, cannot raise the temperature of our bodies beyond certain limits, as it does that of inanimate bodies, because an excess of caloric is carried off by perspiration.

CAROLINE.

But the temperature of the atmosphere, and consequently that of inanimate bodies, is surely never so high as that of animal heat?

MRS. B.

I beg your pardon. Frequently in the East and West Indies, and sometimes in the southern parts of Europe, the atmosphere is above ninety-eight degrees, which is the common temperature of animal heat. Indeed, even in this country, it occasionally happens that the sun's rays, setting full on an object, elevate its temperature above that point.

In illustration of the power which our bodies have to resist the effects of external heat, Sir Charles Blagden, with some other gentlemen, made several very curious experiments. He remained for some time in an oven heated to a temperature not much inferior to that of boiling water, without suffering any other inconvenience than a profuse perspiration, which he supported by drinking plentifully.

EMILY.

He could scarcely consider the perspiration as an inconvenience, since it saved him from being baked by giving vent to the excess of caloric.

CAROLINE.

I always thought, I confess, that it was from the heat of the perspiration that we suffered in summer.

MRS. B.

You now find that you are quite mistaken. Whenever evaporation takes place, cold, you know, is produced in consequence of a quantity of caloric being carried off in a latent state; this is the case with perspiration, and it is in this way that it affords relief. It is on that account also that we are so apt to catch cold, when in a state of profuse perspiration. It is for the same reason that tea is often refreshing in summer, though it appears to heat you at the moment you drink it.

EMILY.

And in winter, on the contrary, tea is pleasant on account of its heat.

MRS. B.

Yes; for we have then rather to guard against a deficiency than an excess of caloric, and you do not find that tea will excite perspiration in winter, unless after dancing, or any other violent exercise.

CAROLINE.

What is the reason that it is dangerous to eat ice after dancing, or to drink any thing cold when one is very hot?

MRS. B.

Because the loss of heat arising from the perspiration, conjointly with the chill occasioned by the cold draught, produce more cold than can be borne with safety, unless you continue to use the same exercise after drinking that you did before; for the heat occasioned by the exercise will counteract the effects of the cold drink, and the danger will be removed. You may, however, contrary to the common notion, consider it as a rule, that cold liquids may, at all times, be drunk with perfect safety, however hot you may feel, provided you are not at the moment in a state of great perspiration, and on condition that you keep yourself in gentle exercise afterwards.

EMILY.

But since we are furnished with such resources against the extremes of heat or cold, I should have thought that all climates would have been equally wholesome.

MRS. B.

That is true, in a certain degree, with regard to those who have been accustomed to them from birth; for we find that the natives of those climates, which we consider as most deleterious, are as healthy as ourselves; and if such climates are unwholesome to those who are habituated to a more moderate temperature, it is because the animal economy does not easily accustom itself to considerable changes.

CAROLINE.

But pray, Mrs. B., if the circulation preserves the body of an uniform temperature, how does it happen that animals are sometimes frozen?

MRS. B.

Because, if more heat be carried off by the atmosphere than the circulation can supply, the cold will finally prevail, the heart will cease to beat, and the animal will be frozen. And, likewise, if the body remained long exposed to a degree of heat, greater than the perspiration could carry off, it would at last lose the power of resisting its destructive influence.

CAROLINE.

Fish, I suppose, have no animal heat, but only partake of the temperature of the water in which they live?

EMILY.

And their coldness, no doubt, proceeds from their not breathing?

MRS. B.

All kinds of fish breathe more or less, though in a much smaller degree than land animals. Nor are they entirely destitute of animal heat, though, for the same reason, they are much colder than other creatures. They have comparatively but a very small quantity of blood, therefore but very little oxygen is required, and a proportionally small quantity of animal heat is generated.

CAROLINE.

But how can fish breathe under water?

MRS. B.

They breathe by means of the air which is dissolved in the water, and if you put them into water deprived of air by boiling, they are soon suffocated.

If a fish is confined in a vessel of water closed from the air, it soon dies; and any fish put in afterwards would be killed immediately, as all the air had been previously consumed.

CAROLINE.

Are there any species of animals that breathe more than we do?

MRS. B.

Yes; birds, of all animals, breathe the greatest quantity of air in proportion to their size; and it is to this that they are supposed to owe the peculiar firmness and strength of their muscles, by which they are enabled to support the violent exertion of flying.

This difference between birds and fish, which may be considered as the two extremes of the scale of muscular strength, is well worth observing. Birds residing constantly in the atmosphere, surrounded by oxygen, and respiring it in greater proportions than any other species of animals, are endowed with a superior degree of muscular strength, whilst the muscles of fish, on the contrary, are flaccid and oily; these animals are comparatively feeble in their motions, and their temperature is scarcely above that of the water in which they live. This is, in all probability, owing to their imperfect respiration; the quantity of hydrogen and carbon, that is in consequence accumulated in their bodies, forms the oil which is so strongly characteristic of that species of animals, and which relaxes and softens the small quantity of fibrine which their muscles contain.

CAROLINE.

But, Mrs. B., there are some species of birds that frequent both elements, as, for instance, ducks and other water fowl. Of what nature is the flesh of these?

MRS. B.

Such birds, in general, make but little use of their wings; if they fly, it is but feebly, and only to a short distance. Their flesh, too, partakes of the oily nature, and even in taste sometimes resembles that of fish. This is the case not only with the various kinds of water fowls, but with all other amphibious animals, as the otter, the crocodile, the lizard, &c.

CAROLINE.

And what is the reason that reptiles are so deficient in muscular strength?

MRS. B.

It is because they usually live under ground, and seldom come into the atmosphere. They have imperfect, and sometimes no discernible organs of respiration; they partake therefore of the soft oily nature of fish; indeed, many of them are amphibious, as frogs, toads, and snakes, and very few of them find any difficulty in remaining a length of time under water. Whilst, on the contrary, the insect tribe, that are so strong in proportion to their size, and alert in their motions, partake of the nature of birds, air being their peculiar element, and their organs of respiration being comparatively larger than in other classes of animals.

I have now given you a short account of the principal animal functions. However interesting the subject may appear to you, a fuller investigation of it would, I fear, lead us too far from our object.

EMILY.

Yet I shall not quit it without much regret; for of all the branches of chemistry, it is certainly the most curious and most interesting.

CAROLINE.

But, Mrs. B., I must remind you that you promised to give us some account of the nature of milk.

MRS. B.

True. There are several other animal productions that deserve likewise to be mentioned. We shall begin with milk, which is certainly the most important and the most interesting of all the animal secretions.

Milk, like all other animal substances, ultimately yields by analysis oxygen, hydrogen, carbon, and nitrogen. These are combined in it under the forms of albumen, gelatine, oil, and water. But milk contains, besides, a considerable portion of phosphat of lime, the purposes of which I have already pointed out.

CAROLINE.

Yes; it is this salt which serves to nourish the tender bones of the suckling.

MRS. B.

To reduce milk to its elements, would be a very complicated, as well as useless operation; but this fluid, without any chemical assistance, may be decomposed into three parts, cream, curds, and whey. These constituents of milk have but a very slight affinity for each other, and you find accordingly that cream separates from milk by mere standing. It consists chiefly of oil, which being lighter than the other parts of the milk, gradually rises to the surface. It is of this, you know, that butter is made, which is nothing more than oxygenated cream.

CAROLINE.

Butter, then, is somewhat analogous to the waxy substance formed by the oxygenation of vegetable oils.

MRS. B.

Very much so.

EMILY.

But is the cream oxygenated by churning?

MRS. B.

Its oxygenation commences previous to churning, merely by standing exposed to the atmosphere, from which it absorbs oxygen. The process is afterwards completed by churning; the violent motion which this operation occasions brings every particle of cream in contact with the atmosphere, and thus facilitates its oxygenation.

CAROLINE.

But the effect of churning, I have often observed in the dairy, is to separate the cream into two substances, butter and butter-milk.

MRS. B.

That is to say, in proportion as the oily particles of the cream become oxygenated, they separate from the other constituent parts of the cream in the form of butter. So by churning you produce, on the one hand, butter, or oxygenated oil; and, on the other, butter-milk, or cream deprived of oil. But if you make butter by churning new milk instead of cream, the butter-milk will then be exactly similar in its properties to creamed or skimmed milk.

CAROLINE.

Yet butter-milk is very different from common skimmed milk.

MRS. B.

Because you know it is customary, in order to save time and labour, to make butter from cream alone. In this case, therefore, the butter-milk is deprived of the creamed milk, which contains both the curd and whey. Besides, in consequence of the milk remaining exposed to the atmosphere during the separation of the cream, the latter becomes more or less acid, as well as the butter-milk which it yields in churning.

EMILY.

Why should not the butter be equally acidified by oxygenation?

MRS. B.

Animal oil is not so easily acidified as the other ingredients of milk. Butter, therefore, though usually made of sour cream, is not sour itself, because the oily part of the cream had not been acidified. Butter, however, is susceptible of becoming acid by an excess of oxygen; it is then said to be rancid, and produces the sebacic acid, the same as that which is obtained from fat.

EMILY.

If that be the case, might not rancid butter be sweetened by mixing with it some substance that would take the acid from it?

MRS. B.

This idea has been suggested by Sir H. Davy, who supposes, that if rancid butter were well washed in an alkaline solution, the alkali would separate the acid from the butter.

CAROLINE.

You said just now that creamed milk consisted of curd and whey. Pray how are these separated?

MRS. B.

They may be separated by standing for a certain length of time exposed to the atmosphere; but this decomposition may be almost instantaneously effected by the chemical agency of a variety of substances. Alkalies, rennet*, and indeed almost all animal substances, decompose milk by combining with the curds.

Acids and spirituous liquors, on the other hand, produce a decomposition by combining with the whey. In order, therefore, to obtain the whey pure, rennet, or alkaline substances, must be used to attract the curds from it.

But if it be wished to obtain the curds pure, the whey must be separated by acids, wine, or other spirituous liquors.

[Footnote *: Rennet is the name given to a watery infusion of the coats of the stomach of a sucking calf. Its remarkable efficacy in promoting coagulation is supposed to depend on the gastric juice with which it is impregnated.]

EMILY.

This is a very useful piece of information; for I find white-wine whey, which I sometimes take when I have a cold, extremely heating; now, if the whey were separated by means of an alkali instead of wine, it would not produce that effect.

MRS. B.

Perhaps not. But I would strenuously advise you not to place too much reliance on your slight chemical knowledge in medical matters. I do not know why whey is not separated from curd by rennet, or by an alkali, for the purpose which you mention; but I strongly suspect that there must be some good reason why the preparation by means of wine is generally preferred. I can, however, safely point out to you a method of obtaining whey without either alkali, rennet, or wine; it is by substituting lemon juice, a very small quantity of which will separate it from the curds.

Whey, as an article of diet, is very wholesome, being remarkable light of digestion. But its effect, taken medicinally, is chiefly, I believe, to excite perspiration, by being drunk warm on going to bed.

From whey a substance may be obtained in crystals by evaporation, called sugar of milk. This substance is sweet to the taste, and in its composition is so analogous to common sugar, that it is susceptible of undergoing the vinous fermentation.

CAROLINE.

Why then is not wine, or alcohol, made from whey?

MRS. B.

The quantity of sugar contained in milk is so trifling, that it can hardly answer that purpose. I have heard of only one instance of its being used for the production of a spirituous liquor, and this is by the Tartan Arabs; their abundance of horses, as well as their scarcity of fruits, has introduced the fermentation of mares' milk, by which they produce a liquor called koumiss. Whey is likewise susceptible of being acidified by combining with oxygen from the atmosphere. It then produces the lactic acid, which you may recollect is mentioned amongst the animal acids, as the acid of milk.

Let us now see what are the properties of curds.

EMILY.

I know that they are made into cheese; but I have heard that for that purpose they are separated from the whey by rennet, and yet this you have just told us is not the method of obtaining pure curds?

MRS. B.

Nor are pure curds so well adapted for the formation of cheese. For the nature and flavour of the cheese depend, in a great measure, upon the cream or oily matter which is left in the curds; so that if every particle of cream be removed from the curds, the cheese is scarcely eatable. Rich cheeses, such as cream and Stilton cheeses, derive their excellence from the quantity, as well as the quality, of the cream that enters into their composition.

CAROLINE.

I had no idea that milk was such an interesting compound. In many respects there appears to me to be a very striking analogy between milk and the contents of an egg, both in respect to their nature and their use. They are, each of them, composed of the various substances necessary for the nourishment of the young animal, and equally destined for that purpose.

MRS. B.

There is, however, a very essential difference. The young animal is formed, as well as nourished, by the contents of the egg-shell; whilst milk serves as nutriment to the suckling, only after it is born.

There are several peculiar animal substances which do not enter into the general enumeration of animal compounds, and which, however, deserve to be mentioned.

Spermaceti is of this class; it is a kind of oily substance obtained from the head of the whale, which, however, must undergo a certain preparation before it is in a fit state to be made into candles. It is not much more combustible than tallow, but it is pleasanter to burn, as it is less fusible and less greasy.

Ambergris is another peculiar substance derived from a species of whale. It is, however, seldom obtained from the animal itself, but is generally found floating on the surface of the sea.

Wax, you know, is a concrete oil, the peculiar product of the bee, part of the constituents of which may probably be derived from flowers, but so prepared by the organs of the bee, and so mixed with its own substance, as to be decidedly an animal product. Bees' wax is naturally of a yellow colour, but it is bleached by long exposure to the atmosphere, or may be instantaneously whitened by the oxy-muriatic acid. The combustion of wax is far more perfect than that of tallow, and consequently produces a greater quantity of light and heat.

Lac is a substance very similar to wax in the manner of its formation; it is the product of an insect, which collects its ingredients from flowers, apparently for the purpose of protecting its eggs from injury. It is formed into cells, fabricated with as much skill as those of the honey-comb, but differently arranged. The principal use of lac is in the manufacture of sealing-wax, and in making varnishes and lacquers.

Musk, civet, and castor, are other particular productions, from different species of quadrupeds. The two first are very powerful perfumes; the latter has a nauseous smell and taste, and is only used medicinally.

CAROLINE.

Is it from this substance that castor oil is obtained?

MRS. B.

No. Far from it, for castor oil is a vegetable oil, expressed from the seeds of a particular plant; and has not the least resemblance to the medicinal substance obtained from the castor.

Silk is a peculiar secretion of the silk-worm, with which it builds its nest or cocoon. This insect was originally brought to Europe from China. Silk, in its chemical nature, is very similar to the hair and wool of animals; whilst in the insect it is a fluid, which is coagulated, apparently by uniting with oxygen, as soon as it comes in contact with the air. The moth of the silk-worm ejects a liquor which appears to contain a particular acid, called bombic, the properties of which are but very little known.

EMILY.

Before we conclude the subject of the animal economy, shall we not learn by what steps dead animals return to their elementary state?

MRS. B.

Animal matter, although the most complicated of all natural substances, returns to its elementary state by one single spontaneous process, the putrid fermentation. By this, the albumen, fibrine, &c. are slowly reduced to the state of oxygen, hydrogen, nitrogen, and carbon; and thus the circle of changes through which these principles have passed is finally completed. They first quitted their elementary form, or their combination with unorganised matter, to enter into the vegetable system. Hence they were transmitted to the animal kingdom; and from this they return, again to their primitive simplicity, soon to re-enter the sphere of organised existence.

When all the circumstances necessary to produce fermentation do not take place, animal, like vegetable matter, is liable to a partial or imperfect decomposition, which converts it into a combustible substance very like spermaceti. I dare say that Caroline, who is so fond of analogies, will consider this as a kind of animal bitumen.

CAROLINE.

And why should I not, since the processes which produce these substances are so similar?

MRS. B.

There is, however, one considerable difference; the state of bitumen seems permanent, whilst that of animal substances, thus imperfectly decomposed, is only transient; and unless precautions be taken to preserve them in that state, a total dissolution infallibly ensues. This circumstance, of the occasional conversion of animal matter into a kind of spermaceti, is of late discovery. A manufacture has in consequence been established near Bristol, in which, by exposing the carcases of horses and other animals for a length of time under water, the muscular parts are converted into this spermaceti-like substance. The bones afterwards undergo a different process to produce hartshorn, or, more properly, ammonia, and phosphorus; and the skin is prepared for leather.

Thus art contrives to enlarge the sphere of useful purposes, for which the elements were intended by nature; and the productions of the several kingdoms are frequently arrested in their course, and variously modified, by human skill, which compels them to contribute, under new forms, to the necessities or luxuries of man.

But all that we enjoy, whether produced by the spontaneous operations of nature, or the ingenious efforts of art, proceed alike from the goodness of Providence. —To GOD alone man owes the admirable faculties which enable him to improve and modify the productions of nature, no less than those productions themselves. In contemplating the works of the creation, or studying the inventions of art, let us, therefore, never forget the Divine Source from which they proceed; and thus every acquisition of knowledge will prove a lesson of piety and virtue.

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