Chap. 32.] Of Animal Heat. 34! 



medium, the venous blood acquires a deeper hue^. 

 that a greater quantity of air is vitiated in a given 

 time, and, confequently, that more heat is abforbed by 

 the blood. It appears, therefore, that nature has in 

 this, as in many other inftances, connected the occafion 

 with the means of ivipplying it. Since, therefore, it is 

 proved, that heat is abforbed from the air in refpiration, 

 and fince the quantity abforbed is not only adequate >to 

 the purpofe, but proportioned to the occafion, we may 

 confider ourfelves as greacly indebted to Dr. Crawford 

 "for having thrown great light on a moft important 

 function, but which muft ftill be enumerated, on fome 

 accounts, among thofe obfcure proceffes of nature, on 

 which human ingenuity may exert its powers, but 

 which it can probably never completely reveal. 



The analogy between combuftion and refpiration - 

 cannot fail to ftrike every perfon acquainted with the 

 nature of thefe procefles. Air, in which a candle has 

 remained till extinguished, is incapable of fupporting 

 animal life. On the other hand, air, in which an ani- 

 mal has remained till it expired, will not fupport the 

 flame of a candle. In both thefe cafes heat is -loft by 

 the air employed; becaufe gaffeous compounds are 

 formed which have lefs capacity for heat than oxygen 

 gas pofiefies. There is one remarkable difference, 

 however, between combuftion and refpiration j in. com- 

 buftion, the. heat derived from the air becomes imme- 

 diately perceptible, and frequently rifes to an intenfc 

 degree. In refpiration, however, the heat is gradually 

 evolved, i.n confequencq _of an admirable law of the 

 animal economy, which has already been adverted 

 to in explaining Dr. Crawford's theory of animal heat. 



What oily and fpirituous fubftances are to a lamp 



they are alfo to the animal body, and no animal is 



Capable of fubfifting for a qonfiderable time without 



Z 3 food 



