GENERAL INTRODUCTION 173 



and animal. These many kinds of substances may be 

 considered in three different classes, elements, com- 

 pounds, and mixtures. 



198. Elements. Of this great number of substances 

 there will of course be some that are composed of sev- 

 eral simpler ones. These simpler ones may, in turn, be 

 made of others that are still more simple. But clearly 

 we cannot go on, without limit, breaking up each of 

 these simple substances into simpler ones; that is, we 

 must soon reach substances that are perfectly simple 

 that cannot be broken up into anything else. Such sub- 

 stances, that cannot be divided into anything else, are called 

 elements. They are absolutely pure, each composed of 

 only the one kind ; the smallest particle of an element 

 would be of just the same nature as a large mass of it. 



Now with these facts clearly in mind, we shall easily 

 see that elements cannot be made, as some substances are, 

 by composition, since each is composed of itself only. 

 The elements only occur ; that is, they are found on 

 earth, sometimes in a pure state but more often united 

 with other elements. They may be separated from these 

 other elements by different methods, called analysis. 



Every substance, then, is made of elements ; either of 

 one alone or of two or more together. Nearly eighty 

 elements have been discovered and named. Most of 

 these are uncommon. Hardly a dozen occur in very 

 large quantities. Of the common elements, oxygen, 

 hydrogen, nitrogen, and chlorine are gases ; mercury is 

 the only familiar liquid ; of solids, there are carbon, 

 sulphur, phosphorus, and some metals ( 212). 



