272 ITS BEAKING ON MEDICAL PRACTICE. 



LAVOISIER'S THEORY OF ANIMAL HEAT. 



The doctrine of '^innate" beat, tauglit by nippocrates and Galen, rnled 

 in medicine for fifteen hundred years after Galen's death, until it received 

 its death-blow IVom the genius of Lavoisier, who demonstrated in his 

 celebrated memoir read before the French Academy of Sciences in 1783, 

 that the source of animal heat is to be found in the combustion of the 

 carbon of the body by the oxygen of the air received into the lungs by 

 respiration. Lavoisier's experiments were repeated and confirmed in 

 1822 by Dulong and Despretz, and have formed the starting-point for 

 all modern investigations on the relation of food to work. As already 

 stated, the work done by food in the body may be divided into — 



1. The work of growth. 



2. The work of animal heat. 



3. Mechanical work. 



4. Vital work. 



Lavoisier arranged his experiments so as to exclude almost all the 

 foregoing kinds of work except that of animal heat. A Guineivpig 

 was placed under a bell-glass inverted over a surface of mercury, and a 

 current of fresh air was allowed to circulate through the apparatus, 

 being passed at its final exit through tubes containing caustic potash, 

 which arrested the carbonic acid produced by the animal. In this 

 manner it was easy to ascertain the carbonic acid excreted by the 

 increase in weight of the tubes of caustic potash during the experiment. 



Lavoisier found that his Guinea pig, in ten hours, burned, on the 

 average, 3.333 grams of carbon ; and this quantity of carbon he esti- 

 mated, from other experiments, as capable of melting 32G.75 grams 

 of ice at the freezing temperature. The same Guinea pig was then 

 placed in an ice-calorimeter, and left in it for ten hours, during which 

 time the heat of its body was found to have melted 402.27 grams of ice 

 at the freezing temperature. 



If we use, instead of the coefficient of combustion of carbon employed 

 by Lavoisier, that now generally adopted from the experiments of Favre 

 and Silber maun,, the quantity of melted ice represented by 3^ grams of 

 carbon would become 304.78 grams instead of 326.75 grams. We are, 

 therefore, entitled to say that the heat of combustion of expired carbon 

 determined by Lavoisier is equal to 



^^^ = 90.68 per cent. 

 402.27 



of the animal heat developed, which is regarded as 100 parts. 



Two years later, in 1785, Lavoisier lai€l before the Royal Society of 

 Medicine of Paris, an account of further experiments, also conducted on 

 the breathing of Guinea pigs, by which he showed that of 100 parts of 

 oxygen absorbed by those animals, 81 only reapi^eared in the form of 

 carbonic acid, and 19 i^arts disappeared altogether. Lavoisier consid- 

 ered that these 19 parts of oxygen were employed in the body in the 

 combustion of hydrogen, the product of such combustion being water. 



