202 Lord Kelvin : Animal Thermostat. 



temperature at which that fluid is no longer capable of com- 

 bining- with phlogiston, and at which it must of course cease 

 to give off heat ? It was partly with a view to investigate 

 the truth of this opinion that Dr. Crawford was led to make 

 the experiments recited above/'' 



These views of Dr. Crawford and u his worthy and ingem- 

 ous friend Mr. Wilson*, of Glasgow, 5 ' express, about as well 

 as it was possible to express before the chemical discoveries 

 of carbonic acid and oxygen, the now well-known truth that 

 oxygen carried along with, but not chemically combined with, 

 food in the arteries, combines with the carried food in the 

 capillaries or surrounding tissues in the outlying regions, 

 and yields carbonic acid to the returning venous blood : this 

 carbonic acid giving the venous blood its darker colour, and 

 being ultimately rejected from the blood and from the body 

 through the langs, and carried away in the breath. Craw- 

 ford's very important discovery that the venous blood of a 

 dog which had been kept for some time in a hot- water bath 

 at 112° Fahr. was almost undistinguishable from its arterial 

 blood proves that it contained much less than the normal 

 amount of carbonic acid, and that it may even have contained 

 no carbonic acid at all. Chemical analysis of the breath in 

 the circumstances would be most interesting ; and it is to be 

 hoped that this chemical experiment will be tried on men. 

 It seems indeed, with our present want of experimental 

 knowledge of animal thermodynamics, and with such know- 

 ledge as we have of physical thermodynamics, that the breath 

 of an animal kept for a considerable time in a hot-water 

 bath above the natural temperature of its body may be found 

 to contain no carbonic acid at all. But even this would not 

 explain the generation of cold which Dr. Crawford so clearly 

 and pertinaciously pointed out. Very careful experimenting 

 ought to be performed to ascertain whether or not there is a 

 surplus of oxygen in the breath ; more oxygen breathed out 

 than taken in. If this is found to be the case, the animal 

 cold would be explained by deoxidation (unburning) of 

 matter within the body. If this matter is wholly or partly 

 water, free hydrogen might be found in the breath : or the 

 hydrogen of water left by oxygen might be disposed of in 

 the body, in less highly oxygenated compounds than those 

 existing when animal heat is wanted for keeping up the 

 temperature of the body, or when the body is dynamically 

 doing work. 



* Who, no doubt, was Dr. Alex. Wilson, first Professor of Astronomy 

 in the University of Glasgow (1760-1784) ; best known now for his 

 ingenious views regarding sun-spots. 



