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continued fermentation going on in the bowels of the earth, liberating fire 
was confident, that the oxygen air which was absorbed by the blood (as is proved in Sect. VI.) 
was the true source of animal heat. Struck with the important discovery, he proposed it as the 
subject of his thesis at Cambridge. The professors of physic and of chemistry were pleased at the 
novelty of the opinion, and verjr politely consented to his disputation on that question, so that Dr. 
Thornton first maintained at Cambridge, previous to his receiving his degree in physic in that univer¬ 
sity, in opposition to the opinion of Dr. Crawford, “ that the venal blood in the lungs absorbs from 
“ air not fire, but oxygen, in combination with the matter of heat (oxygen air), and that in the 
“ circuit of the blood through the body, the oxygen , meeting with some superior attraction, is divorced 
“ from its caloric, (the matter of heat), which becoming disengaged .just as an acid discovers its sen- 
“ sible properties, its alkaline basis being withdrawn from it ), so did it assume its well known 
l< active character; and as lineombined fire ever tends to form an equilibrium, or equal temperature 
“ with the substances around, by pervading the body, it became the source of vital or animal 
“ HEAT. 
Animal heat, therefore, appears to be a gentle combustion :— and an animal in many respects may 
be compared to a burning lamp ; the heat produced in both cases arising from the same cause. 
If an antmal be placed in an exhausted receiver of an air pump it quickly expires; in similar cir¬ 
cumstances a burning lamp goes out. If an animal be not supplied with fresh air it dies, and its 
heat is extinguished; so it is with the lamp. The air breathed by animals is diminished in quan¬ 
tity ; so it is by the burning of the lamp. A certain quantity of air supports an animal for a cer¬ 
tain time, but no longer; so it will keep up the flame of the lamp, for a certain time only. The 
air in which a lamp has burnt out destroys animal life', so the air that the animal hath breathed, 
puts out the lamp. Fixed, azotic, and inflammable airs, destroy animals; so likewise do they 
extinguish the lamp. 
A living animal and a burning lamp, therefore, exactly agree in requiring the same kind of 
air to support them, and in producing the same effects upon the air, to which they are exposed. 
But they do not resemble each other only in producing heat, and requiring the same kind of 
air: for if an animal hath not fresh supplies of food, as well as air, after a certain time it dies, 
and becomes cold; just in the same manner as the lamp dies out, if not duly supplied with oil. 
Since then that part of the air destroyed by respiration is the same as that destroyed by com- 
bustion; and since the ultimate effect is the same in both operations, that is, the production of 
heat, is it not reasonable to think, that the food affords to the animal principles alike attractive 
of oxygen, and disengaging heat, as the oil affords to the lamp? For since the effects are the 
same, the cause must be so too. Oil, therefore, affords the principle attractive of oxygen to 
the lamp: and, consequently, the food of animals suppoi'ts the generation of heat, by supplying 
to the animal body those principles which are attractive of oxygen, the base of vital air. 
The chemical analysis of such substances as are to support animal life confirms this opinion; for 
no substance affords proper nourishment, which contains not principles that readily combine with 
oxygen; and the instantaneous support, and refreshment, perceived by those, who are much exhausted, 
upon taking into the stomach certain inflammable substances, as diluted spirits, &c. depend upon the 
same principle. Very different matters, therefore, will support animal life, if they contain princi¬ 
ples, separable by the animal process, that have an affinity with oxygen. 
To prove that animal heat arises from the decomposition of vital air by those substances which 
attract oxygen, we have the following very striking fact. 
Dr. Beddoes, the justly celebrated professor of chemistry at Oxford, respired at times for seven 
weeks air of a much higher than the ordinary standard, and commonly such as contained almost equal 
parts of oxygen and common air. He relates, in his letter to Dr. Darwin, “ that he felt that agreeable 
f<r glow, and lightness of the chest, which has been described by Dr. Priestley and others. In no long 
<<r time,” he says, “ I observed in myself a remarkable power of sustaining cold. Except one or two 
te evenings 1 never once experienced the sensation of chilliness, though cold easterly winds prevailed, 
“ during great part of the time I inspired the super-oxygenated air . I was not only able,” he adds, 
2 o 
