INTRODUCTION, XXIX 



of the same chemical substances always produces the same amount of 

 kinetic energy, i.e., of heat. 



A person when fasting, experiences after a certain time, the dis- 

 agreeable feeling of exhaustion of his reserve of potential energy, 

 hunger sets in, and he takes food. All food for the animal kingdom is 

 obtained, either directly or indirectly, from the vegetable kingdom. Even 

 carnivora, which eat the flesh of other animals, only eat organised 

 matter which has been formed from vegetable food. The existence of 

 the animal kingdom presupposes the existence of the vegetable 

 kingdom. 



All substances, therefore, necessary for the food of animals occur in 

 vegetables. Besides water and the inorganic constituents, plants 

 contain, amongst other organic compounds, the following three chief re- 

 presentatives of food-stuffs fats, carbohydrates, and proteids. 



All these contain stores of potential energy, in virtue of their com- 

 plex chemical constitution. 



The fats contain :- j Cn !^-< H) = f f* add9 } ( 251). 



( + C 3 H 5 (OH) 3 = glycerine j 



The carbohydrates contain : C 6 H 10 5 . . . ( 252). 



f C. 51-5-54-5 ' 



H. 6-9- 7-3 



The proteids contain per cent.: { N. 1 5-2-17-0 



I 0. 20-9-23-5 



[ S. 0-3- 2-0 J 



A man, who takes a certain amount of this food adds thereto oxygen 

 from the air in the process of respiration. Combustion or oxidation 

 then takes place, whereby chemically potential energy is transformed 

 into heat. 



It is evident, that the products of this combustion must be bodies of 

 simpler constitution bodies with less complex arrangement of their 

 atoms, with the greatest possible saturation of the affinities of their 

 atoms, of greater stability, partly rich^in 0, and possessing either no 

 potential energy, or only very little. These bodies are carbonic acid 

 (carbon dioxide), C0 2 ; water, H 2 0; and as the chief representative of 

 the nitrogenous excreta, urea (CO(NH 2 ) 2 ), which has still a small 

 amount of potential energy, but which outside the body readily splits 

 into C0 2 and ammonia (NH 3 ). 



The human body is an organism in which, by the phenomena of 

 oxidation, the complex nutritive materials of the vegetable kingdom, 

 which are highly charged with potential energy, are transformed into 

 simple chemical bodies, whereby the potential energy is transformed 



