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.