Having, with the most scrupulous attention, ascertained the _latent_, or, as he calls it, the _absolute_ heat of blood, and also that of the aliments of which it is composed, he finds that it contains more than could have been derived from _them_. Also finding that the absolute heat of arterial blood exceeds that of venous blood, in the proportion of 11-1/2 to 10, he concludes that it derives its heat from the air respired in the lungs, and that it parts with this _latent_ heat, so that it becomes sensible, in the course of its circulation, in which it becomes loaded with phlogiston, which it communicates to the air in the lungs. That this heat is furnished by the _air_, he proves, by finding, that that which we inspire contains more heat than that which we expire, or than the aqueous humor which we expire along with it, in a very considerable proportion; so that if the heat contained in the pure air did not become latent in the blood, it would raise its temperature higher than that of red-hot iron. And again, if the venous blood, in being converted into arterial blood, did not receive a supply of latent heat from the air, its temperature would fall from 96 to 104 below 0 in Fahrenheit's thermometer. That the heat procured by combustion has the same source, viz. the dephlogisticated air that is decomposed in the process, is generally allowed; and Dr. Crawford finds, that when equal portions of air are altered by the respiration of a Guinea pig, or by the burning of charcoal, the quantity of heat communicated by the two processes is nearly equal. The following facts are also alleged in favour of his theory. Whereas animals which have much red blood, and respire much, have the power of keeping themselves in a temperature considerably higher than that of the surrounding atmosphere, other animals, as _frogs_ and _serpents_, are nearly of the same temperature with it; and those animals which have the largest respiratory organs, as birds, are the warmest; also the degree of heat is in some measure proportionable to the quantity of air that is respired in a given time, as in violent exercise. It has been observed, that animals in a medium hotter than the blood have a power of preserving themselves in the same temperature. In this case the heat is probably carried off by perspiration, while the blood ceases to receive, or give out, any heat; and Dr. Crawford finds, that when an animal is placed in a warm medium the colour of the venous blood approaches nearer to that of the arterial than when it is placed in a colder medium; and also, that it phlogisticates the air less than in the former case; so that in these circumstances respiration has not the same effect that it has in a colder temperature, in giving the body an additional quantity of heat; which is an excellent provision in nature, as the heat is not wanted, but, on the contrary, would prove inconvenient. LECTURE XXXII. _Of Light._