correct ideas began to prevail respecting the cause of animal heat. After he had discovered that the same kind of aeriform fluid which is produced by the burning of fuel is expired from the lungs, a strict connexion seemed to be established between the processes of combustion and respiration. He was hence led to infer, as Mayow had previously done, that respiration is a species of combustion, and that the heat thus extricated is employed in preserving the temperature of the animal body above that of the surrounding medium. But Black's explanation of the cause of animal heat, although extremely ingenious, and founded upon what appeared to be correct principles, was liable to an objection which prevented it from being generally received. It was said, that if the lungs are the seat of the supposed combustion, or the focus whence the heat radiates to all parts of the body, their temperature must be much superior to that of the other organs, and in short, that they would be altogether incapable of supporting the degree of heat, to which they would be necessarily exposed. We do not find that Black made any attempt to repel the objection, and we may presume that he conceived it so formidable, as to have induced him altogether to relinquish the hypothesis.

8 Although there appears to be no doubt that Black applied his discovery of the formation of carbonic acid in respiration to explain the production of animal heat, we do not find any reference to it in his lectures, as they were afterwards published by Robison. We may, however, conclude that he announced the hypothesis in his lectures, and that it became generally known through this medium; see the Remarks of Menzies on Respiration, p. 35; Murray, Chem. v. iv. p. 571; Thomson, Chem.

Lavoisier now entered upon his researches into the nature of combustion and other analogous ope

v. iv. p. 630; and Ellis, Inquiry, p. 234; Mr. Ellis says, that the objection referred to in the text was urged by Cullen; also Robison in his preface, p. 53. Probably Cullen, in the following passage in his "Institutions," § 268. may refer to Black's hypothesis. "We take no notice of the suppositions which have been made of the generating powers being confined to certain small portions of the system only. These suppositions give no relief in the general theory: and they are not supported by any particular evidence. The breathing animals are the warmest; but that they are warmer because they breathe, is not more probable, than that they breathe because they are warmer." Leslie's Treatise on Animal Heat contains an account of the popular objections to Black's doctrine; his own hypothesis is that phlogiston is naturally developed by the vessels and gene rates heat. Dr. Black, with the modesty and scrupulous regard. to the literary rights of his contemporaries, which formed so distinguished a trait in his character, when speaking of Crawford's hypothesis of the different capacities of bodies, says, that he applied it "to explain the heat maintained in the bodies of animals," without any allusion to his own opinions; Lect. v. 2. p. 205, 6. It was about this period of the investigation that Franklin threw out a conjecture on the source of animal heat, that the matter of heat was taken into the body along with the food, and was set at liberty during the successive changes which the aliment afterwards experiences; Works, v. ii. p. 79, 125. This hypothesis was subsequently taken up by Rigby and dilated into a Treatise; see sect. 1. of his Work on Animal Heat. Richerand, to a certain extent, adopts the same view of the subject, but his remarks are vague and diffuse; Physiol. § 79. p. 214. I may remark, that an opinion something similar to this had been previously formed by Descartes; he says that the change which the food undergoes in the stomach produces heat, in the same manner as when water is poured upon lime, or aqua fortis upon metals; De Homine, p. 7. Hunter also

râtions, and after repeating and verifying many of the experiments of Black and Priestley, he pointed out more clearly than they had done, the exact nature of the change which is produced upon the air by the action of the lungs. He found a perfect similarity between the results which he obtained from the combustion of charcoal and the products of respiration, and as the evolution of heat is the necessary consequence of the former operation, he was induced to draw the same inference that had been previously made by Black, that heat must be generated by the formation of carbonic acid in the lungs. He does not appear, at least in the first instance, to have felt the objection that had been urged against the doctrine, but brings it forwards as a correct deduction from acknowledged facts, and as a satisfactory method of explaining the phenomena."

considers it probable that the principal source of heat is in the stomach; On the Blood, p. 292; the remark is, however, made in an incidental manner, and he had expressed himself in another passage in the same work dissatisfied with all the theories of animal heat, as not according with the facts; p. 15.

9 Mem. Acad. Scien. pour 1777, p. 599. In tracing the progress of discovery on this subject, it appears somewhat difficult to ascertain the share which Black and Lavoisier respectively bore in establishing the chemical theory of animal heat. Lavoisier, in the above memoir, speaks of the hypothesis as entirely original, and altogether derived from his own experiments, without referring to any preceding authors, and the same statement is made by Seguin; Ann. Chem. t. v. p. 259; see also Fourcroy, Med. Eclair. t. i. p. 56.. 61. There seems, however, to be no doubt, that as far as respects the production of carbonic acid in the lungs, and the consequent evolution of heat, he had been

It was shortly after this period that Crawford commenced his investigations, and illustrated the subject by a series of very elaborate and ingenious experiments; the result of which was the formation of a theory of animal heat, which appeared to account for all the phenomena, and to be founded upon decisive and well established facts, while it was not liable to the objection which had been urged against Black's hypothesis. The scrupulous care with which Crawford endeavoured to avoid every source of inaccuracy, the ingenuousness with which he acknowledged the errors of his first experiments, and the spirit of candour which pervades every part of his work, were calculated to produce a very favourable impression with respect both to the intellectual and the moral qualities of the author, so that his doctrine was very favourably received, and generally embraced by hist contemporaries.

Crawford's theory of animal heat may

be regarded

completely anticipated by Black, while he does not advert to the objection which had been urged against the hypothesis. Although we admit that Robison's zeal for the reputation of his deceased friend may have carried him too far in his observations upon the rival claims of the two philosophers, it seems but too probable that Lavoisier was not always sufficiently correct in appropriating the due share of merit to his contemporaries, a circumstance the more to be lamented, when we consider the very great obligations which he conferred upon science. There are some good observations on this subject in Nicholson's Jour. v. xiv. p. 90, 231, et seq. I may remark that Crawford, who was distinguished for his candour and liberality, does not refer to Black, as having advanced any hypothesis of animal heat, nor does he intimate that his own opinions had been anticipated by Lavoisier.

as founded upon the three following positions; 1. That the air, when taken into the lungs, undergoes the same change as by the combustion of a carbonaceous body, and that consequently, in the same manner as in the combustion of carbon, heat is generated. 2. The same process by which the oxygen of the inspired air is converted into carbonic acid likewise converts the venous into arterial blood, but arterial possessing a greater capacity for heat than venous blood, the heat which would otherwise raise the capacity of the arterialized blood, is employed in saturating its encreased capacity, and in maintaining its temperature at the same degree with the venous. 3. The heat is not therefore actually set at liberty in the lungs, although the arterial contains a greater quantity of absolute heat than the venous blood, but it is during the course of the circulation, when the arterial blood is again venalized, and consequently loses its encreased capacity, that the heat becomes sensible and supports the temperature of the system. The hypothesis which Crawford constructed upon the above positions professed to be the result of direct experiment in all its parts; it removed the objection that had been urged against Black's doctrine, and afforded one of the most interesting and beautiful specimens of the application of physical and chemical reasoning to the animal œconomy that had been ever presented to the world.'

We have a correct and judicious summary of Crawford's theory given us in Prof. de la Rive's elegant inaugural dissertation "De Calori Animali," p. 26, et seq.; and in Dr. Henry's Elements, v. ii. p. 407.