LAW Of THE CONSERVATION OF ENERGY 571 



actual calorific value of a protein to an animal is therefore the same as 

 that of a carbohydrate, both being far inferior in heat-value to the 

 fats. 



The necessary data for the accurate evaluation of the comparison 

 which Lavoisier attempted were by now assembled and the com- 

 parison, when actually carried out by Rubner in 1894, established 

 beyond any doubt the validity of the principle of the Conservation of 

 Energy in the phenomena of life. The experiments were carried out 

 upon a dog, because there existed at that time no calorimeter, of 

 sufficient size to contain a man, which would accurately measure the 

 heat evolved during a period of twenty-four hours. The heat actually 

 imparted by the dog to the calorimeter in twenty-four-hour periods 

 was measured and this was compared with the heat-value of its food 

 computed from the nitrogen in the urine (1 gram Nitrogen = 6.25 

 grams protein = 25.63 calories) and from the output of water and 

 carbon dioxide. The following are the details of his comparisons, the 

 "food" in starvation consisting, of course, of the proteins and fats of 

 the animal's own tissues: 



Number of Heat calculated Heat directly Difference in 



Food. days. from metabolism. determined. percentage. 



-1.42 



2 



Fat . . \ . 5 1510.1 1498.3 -0.97 



2488.0 



Meat and fat 



/ ' ' ' 6 2249 ' 8 2276.91 



\ . . . 7 4780.8 4769.3 / 



iiyr A I V Xf^rtJJ.O ^~t\J.V I 0.42 



iviGo/t \ n-ic\n c\ Ant*f\ *\ t , _i_n j.^? 



When one considers the complexity of these estimations, the multi- 

 tude of factors which participate in determining their outcome, and 

 the elaborate character of the apparatus employed, the coincidence 

 of the calculated and actual output is so exact as to leave no room for 

 doubt that the law of the conservation of energy applies no less to 

 animals than to other machines. The energy which the animal dis- 

 sipates is derived from the combustion of foodstuffs, just as the energy 

 dissipated by a locomotive is derived from the oxidation of its fuel. 

 In the living, as in the inanimate machine, the potential energy of the 

 fuel is released by oxidation and reappears in the form of heat and 

 work. 



An even more exact balance between income and output was how- 

 ever sought for and found by the American investigator, Atwater. 

 The extraordinary degree of accuracy which was attained in his 

 investigations was rendered possible by the invention of the Atwater- 

 Rosa Calorimeter, which was of sufficient capacity to hold a man and 

 yet so technically perfect that when a measured amount of heat was 

 generated within the calorimeter by an electric current, the quantity 

 of heat liberated could be measured to within 0.01 per cent. (Figs. 48 

 and 49). The amount of protein burnt by the subject was estimated 



