An expression borrowed from the Stoical philosophy to express the natural heat of animals, which, as connected with life, has been also called the lamp of life. By the ancient philosophers in general, heat was considered as connected with life, as the peculiar distinguishing property of living animals, or as the effect of divine interposition:
Est Deus in nobis, agitante calescimus ipso. The ideas of Hippocrates on this subject were not very-different; and, though Galen deviated somewhat from his master, no attempts were made to explain its source, till the chemical schools attributed it to effervescence and fermentation; the mechanical philosophers to friction - either of the particles of the blood on each other; of these on the vessels; or of the solid parts themselves. Each of these theories is, however, wholly inconsistent with the appearances, or with the functions, of the animal economy; nor need we in this place enter into arguments to refute opinions which no one at present adopts.
Dr. Franklin supposed that fire, or, in modern language, caloric, was combined with our aliments; and, in the progress of the circulation, when the alimentary substances were decompounded, again separated in an active state: an idea simple and ingenious, in reality the basis of more modern systems. When phlogiston was in fashion, Dr. Black supposed that the air acting on the blood, separated the inflammable principle; and since it was apparently changed in the same manner as it would have been by a burning body, a similar process probably occasioned the change. Dr. Duguid Leslie, in his Thesis, afterwards published separately, opposed this idea; and suggested an opinion not very different from Dr. Franklin's, that the phlogiston contained in all our fluids, was separated during the circulation; and, as in every other circumstance where this principle was separated, heat ensued. This doctrine was, we believe, taught by Dr. Duncan in his class about that time: at least we have good reasons to attribute it to him. Yet each of these opinions must fall with phlogiston; but, though terms alter, we shall find that the principle of each has been retained, and that the same or similar ideas, in different forms, approach very nearly the solution of the problem.
The facts, however, have not been ascertained with accuracy. The heat seems to be almost uniform in every part of the body; and a thermometer under the tongue, in the axilla, in the rectum, in the urethra, and in a sinous ulcer, has pointed to nearly the same degree.
There is, undoubtedly, a difference in the degrees of heat of different persons, and probably indifferent parts. The earlier observations, as the construction of the thermometer was less correct, we shall omit; but, in general, the heat appears to vary from 96° to 98°. Dr. Marline and Dr. Hales found the urine to be 99° and 103° when the skin was 97°. Mr. Hunter observed the heat of the rectum to be 98°, and that of the bulb of the urethra 97°. A thermometer, two inches within the rectum of a dog, was at 100°, in the left ventricle of the heart 101°, in the substance of the liver 100 3°/4, and in the stomach 101°.
De Haen, however, remarks, that if a thermometer be applied under the arm for half a quarter of an hour, its height is 95° or 96°; if for a quarter of an hour, 97°, 98°, and 99°; if for half an hour, 100°, and 101°; if for an hour, 101°, and 102°. This passage has been little noticed, though we suspect it is alluded to by an author, who remarks, that the irritation produced by any body confined so long to a part, must increase the heal above its natural standard. To ascertain the fact, the author of this article put a very accurate and sensible thermometer, made by Dr. Wilson, of Glasgow, under his arm, when in perfect health; and. confining the humerus loosely, so as not to produce the slightest inconvenience, sat down to read. The pulse, as usual with him when sitting still for some hours, sunk to 56% were the same in both arms, and the feelings in each arm continued the same for three hours, during which the experiment lasted. In a quarter of an hour the thermometer was 97% in about an hour 98°, and in two hours after 99. It never rose higher. We must add, that in other trials, when the author's heat has been compared with that of different persons, it has always been found at least a degree lower; so that, perhaps, the real heat of the human body should be considered as 100°. This fact is. we think, of importance, when, with the accuracy of modern chemists, the capacity of the blood for heat in different situations is estimated.
Another fact, for which we are indebted to De Haen, is, that in putrid fevers the heat at the moment of death has been considerably increased, and that it even continued to increase after death, till, if we recollect rightly, it amounted to 104º. To this meagre cata-logue of facts we can only add one other, which we think should be again examined, viz. that the venous blood of the internal organs is hotter than the arterial.
Observation, which ought to have preceded theory, should have furnished many additional circumstances to assist our enquiries; and it would have been of advantage to have ascertained the comparative capacity of heat of the blood in the vena portae and the hepatic veins, perhaps of the blood in the splenic artery and the vasa brevia; nor would it have been wholly useless to have determined the capacity of heat of the different component parts of the vital fluid, with more accuracy than has yet been attempted. We must proceed, however, to explain the modern theories of Dr. Crawford and M. Lavoisier, taking the liberty of changing the age of the former; for, though phlogiston is no more, the language only, so far his theory extends, is changed.