THE LEAF 143 



have seen, only by exposing the compound to the high 

 temperature of burning magnesium. 



However, this case of the decomposition of carbonic 

 acid by means of magnesium cannot serve as an example 

 of a simple process of decomposition, because a com- 

 bination of magnesium with oxygen takes place at the 

 same time. For a long time chemists thought that the 

 decomposition of such stable compounds as carbonic acid 

 and water could not take place without the co-operation 

 of a third body, possessing a still stronger affinity for 

 oxygen, but they have comparatively recently come 

 to the conclusion that the action of heat alone is 

 sufficient to cause the decomposition, or the dissocia- 

 tion of carbonic acid and water. According to modern 

 physics heat is a kind of motion — a rapid, invisible, 

 but palpable vibration of the particles of a body. In 

 heating any compound body to a very high tempera- 

 ture, we bring its particles into such a state of vibration, 

 and loosen them to such an extent, that their mutual 

 coherence finally breaks down, causing decomposition. 

 To give an example : at a very high temperature water 

 vapour does not exist any longer as such, but as a 

 mixture of hydrogen and oxygen. 



During such a process of decomposition energy is 

 absorbed : but what happens to this energy ? It can- 

 not disappear — that would contradict the law of the 

 conservation of energy. It passes during the process 

 into a latent condition of tension. Everyone is familiar 

 with instances of energy stored in a latent state among 

 everyday mechanical phenomena : a sledge-hammer 

 ready to fall upon a stake which is being driven into 

 the soil ; a bow bent ready to shoot an arrow — these are 

 simple cases of energy stored in the form of tension. 

 This expression sounds curious, however, when applied 

 to light or heat. Can such forms of energy as light or 

 heat be stored ? Could I, for instance, get hold of a 

 certain amount of light or heat, such as that evolved 



