ELEMENTARY VITAL PHENOMENA 213 



By chemical energy is understood, as is well known, the capacity 

 of atoms to attract other atoms ; this property has also been 

 termed chemical affinity. Every atom, regarded as isolated, 

 represents accordingly a small magazine of energy. The chemical 

 energy in it is potential so long as the atom has no opportunity 

 to unite hy means of its affinity with another atom. But, as soon 

 as two atoms combine, a part of the potential, corresponding to 

 the strength of their affinities, passes over into kinetic energy and 

 is set free in the form of heat, light, mechanical energy, etc. 

 Since, further, chemical affinity is quantitatively very different in 

 different kinds of atoms, the stronger the combining affinities, the 

 more energy is set free. A chemical compound, must, therefore, 

 contain less potential energy, the stronger the affinities are that 

 have brought together its atoms. Vice versa, if two combined 

 atoms become separated, a certain quantity of kinetic energy is 

 absorbed in the process, and after the separation the same quan- 

 tity appears again in the potential form as the free affinities of 

 the atoms. Thus there is a complete cycle. 



An example will make this relation more evident. Suppose 

 a strong glass cylinder to be inverted over a mercury trough and 

 to contain in a small space free from mercury a gaseous mixture 

 consisting of two-thirds hydrogen and one-third oxygen ; such a 

 mixture consists of molecules whose atoms contain large quantities 

 of potential energy in the form of chemical affinity for one another. 

 If, now, the conditions be made such that the atoms of oxygen 

 and hydrogen can combine, the atoms rush eagerly toward one 

 another, unite and give off to the outside all their stored potential 

 in the form of heat, light, and mechanical energy. A spark 

 appears, the cylinder becomes heated, and the mercury is forcibly 

 driven down. The latter soon rises again, for the vapour that 

 results from the union of the atoms of oxygen and hydrogen 

 becomes condensed with the increasing cooling into water, which 

 finally occupies only a minute space within the cylinder. Thus, 

 in the synthesis of water from hydrogen and oxygen the potential 

 energy of chemical affinity is transformed into kinetic energy and 

 is set free as heat, light, etc. Hence the molecule of water has 

 lost to its environment this quantity of energy, and this can be 

 exactly determined. Vice versa, the atoms of water can be 

 separated again into atoms of hydrogen and oxygen by introducing 

 from outside the same quantity of energy. Electrical energy 

 serves best for this purpose. If an electric current be passed 

 through water, atoms of hydrogen and oxygen are set free at the 

 poles in the same degree as the electrical energy disappears. 

 Hence energy is absorbed in separating the atoms of the water- 

 molecule ; but this energy appears again as the potential of 

 chemical affinity in the free atoms, for, when the free hydrogen 

 and oxygen are brought into combination, kinetic energy is 

 obtained anew, and so on. 



