5 i8 THE POPULAR SCIENCE MONTHLY. 



processes ; for, if the organism is able by their use to keep its own 

 temperature at a higher point than that of its immediate surround- 

 ings, and also to develop mechanical force, then, if not all, at least a 

 majority of these processes, are accompanied by a rise of temperature. 

 As is well known, however, the highest temperatures are obtained by 

 combustion that is, by the combination of other bodies with oxygen. 

 Since oxygen is continually inhaled and consumed by animals during 

 life, we are obliged to consider this as the source of heat and force. 

 We have here a problem which is open to discussion, namely, whether 

 the energy liberated by the combustion was originally contained in 

 the oxygen or in the other substances. It appears as if the latter 

 assumption was generally accepted ; at least, statements are often met 

 with, such as, for instance, that coal contains the heat of the sun 

 which has been stored up during thousands of years. Although we 

 can not, at present, with the means at our disposal, definitely solve 

 this problem, it can at least be shown that the statement has little in 

 its favor. The decomposition of carbonic acid by the influence of 

 the light and heat of the sun is effected in such a manner that the 

 carbon is employed in the formation of the compounds of which the 

 plant is built up, while the oxygen escapes into the atmosphere., Now, 

 we know that solids contain the least energy, because it must be sup- 

 plied to them in the form of heat in order to convert them into the 

 liquid or gaseous state, while, on the contrary, heat must be with- 

 drawn from gases to condense them to liquids or solids. Oxygen is 

 one of the most permanent gases, and must therefore possess an enor- 

 mous amount of energy, while carbon, on the other hand, being one 

 of the most difficultly diffusible and volatile bodies, can only contain 

 a little energy. This makes it extremely probable that the force of 

 the sun, taken up by the plants, is not stored in their bodies, but in 

 the free oxygen of the atmosphere. Hence the latter is to be consid- 

 ered as the inexhaustible source of power on which man and animals 

 draw, and in the carbon we possess a valuable aid for making this 

 energy, contained in the oxygen, available. 



After this digression, let us return to the chemical processes in the 

 animal body. The production of heat and mechanical force, as well 

 as the large quantity of oxygen consumed, indicates that the greater 

 part of these processes must consist of oxidations, and accordingly 

 we find that, among the products of excretion, carbonic acid and water 

 form the principal parts, two bodies which can not be further oxi- 

 dized. Besides these (to which sulphuric acid and phosphoric acid 

 may be added), other substances are excreted in small quantity, which 

 admit of further oxidation, but which have not suffered it in the 

 organism. I will only mention urea, uric acid, and creatinine. All 

 these compounds are products of chemical action, and the question is 

 raised, How are they produced in the body ? to what reactions do they 

 owe their origin ? to a single one, or to a whole series ? Here we 



