4 History of physical Science from [July, 



ture of the animals diminished more rapidly than would have 

 been the case if the respiration had not been kept up at all. 

 From these experiments Mr. Brodie concluded that the heat of 

 animals is not kept up by respiration, but by the energy of the 

 brain and nerves. M. Legallois has made a set of experiments 

 to elucidate this difficult and obscure subject. We conceive it 

 by no means an easy task to make experiments upon living 

 animals from which satisfactory conclusions can be drawn 

 respecting their functions, because they possess the power of 

 accommodating themselves to circumstances. Thus when we 

 put an animal into a close vessel, and oblige him to breathe the 

 same air over and over again, there is reason to believe that the 

 same changes are not produced upon the air that would be 

 produced if the animal were in his usual circumstances, and 

 obliged to draw the same portion of air only once into the lungs. 

 We need not be surprised, therefore, that M. Legallois, notwith- 

 standing all the pains which he took, has not succeeded in 

 throwing much light upon the source of animal heat. 



He observes that when the respiration of a decapitated animal 

 is artificially kept up, the arterial blood is not converted into 

 venous blood during the circulation ; for the colour of the blood 

 in the vena cava is precisely that of arterial blood. This obser- 

 vation will help us to account for the more rapid cooling of the 

 decapitated animals in Mr. Brodie's experiments ; for as the 

 specific heat of arterial blood is greater than that of venous 

 blood, it is obvious that if the change into venous blood does 

 not take place, the waste of heat must be greater than in 

 ordinary cases. 



M. Legallois found that when the respiration of animals is 

 constrained, by .tying them down upon their back their temper- 

 ature diminishes considerably ; so much so, indeed, that if kept 

 a sufficient time in that position, they die of cold. On making 

 comparative experiments on the respiration of rabbits uncon- 

 strained, and fixed down upon their backs, he found that at the 

 temperature of about 50° the quantity of oxygen consumed by 

 them, while fixed upon their back, was considerably less than 

 when breathing at liberty. But when the temperature of the 

 atmosphere was about 70°, no such difference could be observed ; 

 or it was very trifling. The smaller the quantity of oxygen 

 contained originally in the air which these animals were obliged 

 to breathe, the greater was the diminution of temperature which 

 they sustained. When atmospheric air was rarified, and brought 

 to its natural density by a mixture of carbonic acid, the loss of 

 heat which animals breathing such a mixture sustained, was the 

 greatest possible. Yet the quantity of carbonic acid gas in the 

 air did not increase, but rather diminished ; so that it must have 

 been absorbed by the animal. Thus the temperature of a dog 

 obliged to breathe such an atmosphere for three hours sunk 25°, 

 or was reduced from 102° to 78°. The temperature of a cat, in 

 the same circumstances, was reduced from 104° to 8L5°, or 22^°. 



