Conversations on Chemistry Part 108

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 a.n.a.logous, 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 quant.i.ties is certain, and this is the princ.i.p.al, 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 pa.s.sing 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 princ.i.p.al 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 acc.u.mulation 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 amus.e.m.e.nt 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 quant.i.ty 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.