I POTENTIAL AND KINETIC ENERGY 15 



When we say that any combustible body is burnt what we 

 usually mean is that it has combined with oxygen, forming 

 certain products of combustion due to the chemical union 

 of the oxygen with the substance burnt. For instance, when 

 carbon is burnt the product of combustion is carbon dioxide 

 or carbonic acid (C + O.,= CO 2 ) : when hydrogen is burnt, 

 water (H 2 + O = H 2 O;. The products of the slow com- 

 bustion which our own bodies are constantly undergoing 

 are these same two bodies carbon dioxide given off mainly 

 in the air breathed out, and water given off mainly in the 

 form of perspiration and urine together with two com- 

 pounds containing nitrogen, urea (CH 4 N 2 O) and uric acid 

 (C 5 H 4 N 4 O 3 ), both occurring mainly in the urine. In some 

 animals urea and uric acid are replaced by other compounds 

 such as guanin (C 5 H 5 N f) O), but it may be taken as proved 

 that in all living things the products of combustion are 

 carbon dioxide, water, and some nitrogenous substance of 

 simpler constitution than proteids, and allied to the three 

 just mentioned. 



With this breaking down of proteids the vital activities of 

 all organisms are invariably connected. Just as useful 

 mechanical work may be done by the fall of a weight from 

 a given height to the level of the ground, so the work done 

 by the organism is a result of its complex proteids falling, 

 so to speak, to the level of simpler substances. In both 

 instances potential energy or energy of position is converted 

 into kinetic or actual energy. 



In the particular case under consideration we have to rely 

 upon analogy and not upon direct experiment. We may, 

 however, be quite sure that the products of combustion 

 or waste matters of Amoeba include carbon dioxide, water, 

 and some comparatively simple (as compared with proteids) 

 compound of nitrogen. 



