ELEMENTARY VITAL PHENOMENA 216 



when, as in the iodine molecule, the atoms of iodine or, as in the 

 hydrogen molecule, the atoms of hydrogen have a greater affinity 

 for one another than the iodine atoms have for the hydrogen atoms. 

 In these cases more energy becomes absorbed, in order to separate from 

 one another the atoms of the iodine molecule and the atoms of the 

 hydrogen molecule than becomes free when the atoms of iodine and 

 hydrogen unite into a molecule of hydrogen iodide, and, since in 

 every calorimetric experiment the end-result and not the inter- 

 mediate processes come under observation, it is explained why at 

 the end of the reaction there must be an absorption of heat. 

 The reverse is the case in the decomposition-processes accom- 

 panied by an evolution of heat. It is well known that nitroglycerine 

 (glyceryl tri-nitrate), upon being shaken, explodes with an enormous 

 evolution of energy, being decomposed into water, carbonic acid, 

 oxygen and nitrogen. These products of decomposition are not 

 preformed stereochemically in the molecule of nitroglycerine, but 

 they arise from a synthetic rearrangement of the atoms set free by 

 the decomposition. Since the atoms of water, carbonic acid, oxygen 

 and nitrogen, have greater affinities for each other in this arrange- 

 ment than in their position in the nitroglycerine molecule, a small 

 quantity of energy suffices to cause the decomposition of the latter, 

 while from the resulting syntheses an extraordinary quantity of 

 energy becomes free. Hence as the end-result there is an evolution 

 of heat. Therefore, just as in the synthesis of hydrogen iodide, 

 strictly speaking, the absorption of heat is not to be credited to the 

 S3-nthesis, so in the dynamite explosion the evolution of energy does 

 not come in reality from the decomposition of the nitroglycerine 

 molecule. This fact should be clearly understood. But, since, when 

 a synthesis is spoken of, the preceding decomposition is left out of 

 account, and when a decomposition is spoken of, the subsequent 

 synthesis is similarly treated, it is more exact to express the funda- 

 mental law of the transformation of energy in chemical processes 

 in the following form : If in a chemical process affinities become 

 united rather than separated, energy is liberated ; if affinities become 

 separated rather than united, energy is absorbed. 



To return from our excursus, it is clear from the discussion 

 that chemical energy can be introduced into the organism 

 only when the food-stuffs contain affinities for the satisfy- 

 ing of which an opportunity is afforded within the organism. 

 Hence substances must be introduced into the body, which undergo 

 among themselves chemical transformations with the evolution of 

 heat. This takes place in two ways, which we have just 

 become acquainted with, viz., first, by the introduction of simple 

 substances possessing strong affinities, and, second, by the intro- 

 duction into or synthesis within the body of complex compounds 

 which are easily decomposed and, like explosive bodies, furnish 

 decomposition-products that combine synthetically into new 



