PHYSICAL FORCES 129 



certain manner, the rust can again be resolved into 

 oxygen and iron, but no mechanical process will separate 

 them from each other. It is the same gas which, by its 

 chemical action on other substances, causes flame, and all 

 our fires are manifestations of such chemical energy. 



When we mix together, in water, the two finely 

 divided bodies which are often given to us as " effervescing 

 powders/' these two bodies do not merely become liquid 

 and mix. They each become resolved into other sub- 

 stances which then partly unite together (in new 

 combinations to form a new liquid body of a different 

 nature) and partly give rise to an aeriform body a gas 

 which escapes from the liquid in bubbles, so causing 

 the effervescence. This is very different indeed from a 

 mechanical mixture such, for example, as when we mix 

 together two dry powders of different colours. 



The gas, oxygen, which, by uniting with iron, forms 

 " rust," exists in the atmosphere mechanically mixed with 

 the air's other gaseous components. Water, however, 

 can be resolved into that same gas and another gas 

 called hydrogen, and these are not merely mechanically 

 blended. If these two gases be mixed, in due propor- 

 tion, within a carefully closed glass vessel, and an electric 

 spark be made to pass through the mixture, the two 

 gases will entirely disappear and water will be found 

 to exist in their stead. 



Chemical energy will therefore actually transform two 

 or more substances into two or more other substances 

 presenting characters which seem to indicate a quite 

 different nature, with quite different properties. Every 

 substance possesses its own chemical energies, either dor- 

 mant or potential* (as in effervescing powders before they 



* See ante, p. 85. 



