ficient to explain all the phenomena of respiration. Amongst the various modifications proposed, with a view to improve this theory, that suggested by La Grange, Hassenfratz, and some other eminent chemist, appears to be the most important. These philos ophers suppose that the oxygen, which disappears in respiration, is absorbed by the blood, and carried with it into the circulation, during which it gradually combines with the hydrogen and carbone that are succes sively added to the circulation, forming the water and carbonic acid which are expelled from the lungs at each expiration. Thus the process, instead of being completed in the lungs, as the former theory supposes, only begins in that organ, and cotninues throughout the circulation.

According to this theory, the florid colour of arterial blood depends upon the addition of oxygen, so that this colour gradually vanishes as the blood passes from the arterial to the venous state, that is to say, as the oxygen enters into combination with the hydrogen and carbone during circulation.

Caroline. There does not appear to me to be any very essential difference in these two theories, since in both the oxygen purifies the blood by combining with and carrying off the matter which had accumulated in it during circulation.

Mrs. B. Yes; but, in medical, or rather phisiological science, it must be a question of great importance, whether the oxygen actually enters the circulation, or whether it proceeds no further than the lungs.

The blood thus completed, 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 frame, as the muscles, bones, nerves, glands, &c.

Emily. There seems to be a singular analogy be tween the blood of animals, and the sap of vegetables; for each of these fluids contain 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 be tween 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 does these organs derive their respective powers?

Mrs. B. From peculiar organization, the secret of which no one has yet ever 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 immedi ately connected with it, and acts a most important part in the animal economy.

Caroline. Is not this secretion likewise made by ap propriate glands?

Mrs. B. No; it is performed by the extremities of the arteries, which penetrate through the skin and ter minate 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 se creted faster than it can be dissolved, it becomes sensi ble, 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 parts, 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 XXIII.

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 hydrogen and carbone, producing carbonic acid and water.

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 hydrogen and carbone 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 hydrogen and carbone, and produce water and 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 prinpal, if not the only source of ANIMAL HEAT.

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 as it was first proposed by Black and Lavoisier. It is 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 answered by means of the improved theory of respiration which I mentioned last. According to this hypothesis, you recollect, the changes which the blood undergoes in consequence of respiration only begin in the lungs and gradually continue during circulation. Therefore the animal heat, which is the consequence of those changes, likewise begins in the lungs, and afterwards continues during the whole circulation; and heat is thus uniformly diffused throughout every part of the body.

Caroline. More and more admirable !

Mrs. B. Now let me hear whether you can explain how animal heat is produced. You, Caroline, tell me in what manner it is first evolved in the lungs ?

Caroline. Part of the oxygen gas inspired, immediately combines in the lungs with the loose carbone

and hydrogen of the venous blood; and the caloric evolved during this combination, becomes animal heat.

Mrs. B. Very well; but you must observe, that the whole of the oxygen inspired at a breath is not consumed by one respiration: a considerable part of it is expired, so that we may breathe the same portion of air several times before the whole of the oxygen is expended.-Now, Emily, will you explain to me in what manner an uniform degree of heat is kept up throughout the body?

Emily. A quantity of oxygen enters into the circulation during which it gradually combines with the hydrogen and carbone of the blood, thus producing a constant disengagement of heat throughout every part of the body.

Mrs. B. Very well, indeed. You have in a few words stated nearly all that can be said on the subject. I must, however, mention another circumstance which may contribute to account for the gradual evolution of animal heat. It appears, from some experiments, that the blood, in consequence of the successive changes it undergoes during circulation (by which it is gradually` converted from arterial into venous blood), has its capacity for caloric diminished. What must be the consequence of this?

Emily. That heat, of course, must be disengaged. Mrs. B. Exactly so; and thus an additional quantity of animal heat must be generated. However, the heat produced in this way is but trifling, and could only account for a very small portion of the animal temperature.

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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

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