128 ELEMENTS OF SCIENCE 



CHEMICAL ENERGY.* As is manifest even to children, 

 a number of different substances exist, such as sand, 

 stones of different sorts, diamond?, glass, matter known 

 as " salts" of various kinds, silver, gold, iron, and other 

 metals, as well as water, quicksilver, air, the gas which 

 we burn, &c. &c. 



Most of these bodies can be " resolved," by one or 

 another process, into other substances which appear to 

 have composed them, and this appearance is greatly 

 strengthened by the fact that the substances so resolved 

 can often be reproduced by the bringing together, under 

 certain conditions, of the matters into which they were 

 previously resolved. At first it would seem as if such 

 substances have been merely mixed, and that their com- 

 ponent parts can be disentangled and then mixed again. 

 Such, however, does not seem to be the case. 



We all know how readily iron rusts when exposed to 

 the air. But the rust of iron is not iron, nor can we 

 affirm it to be iron mechanically mixed with something else 

 so as to be able, by another mechanical process, to be 

 separated from it again. Iron is one substance, iron rust 

 is another and diverse substance. It is a thing of a differ- 

 ent nature, with a number of qualities quite distinct from 

 those of anything which may, with the iron, have contri- 

 buted to form it to form the rust. And another sub- 

 stance has so contributed a gas contained in the air and 

 known as oxygen. This gas, by acting on the iron at its 

 surface, coalesces with it to form the new substance, 

 " iron rust," or what chemists call oxide oj iron, and the 

 process itself is called oxidation. By heating it in a 



* Some additional information about chemistry will be found 

 in the next chapter (pp. 138 to 145), wherein various properties 

 of minerals are considered, 



