18 



would never say that this compound contained the simple sub- 

 stances iron and sulphur. If he did, we should understand him to 

 mean that it was a mixture, and we should expect parts of the 

 material to be magnetic like iron, and other parts to be yellow and 

 soluble in carbon disulphide, which is not the case. In the same 

 way the name of an element (such as iron) is applied both to 

 the material in combination and to the free substance. Thus 

 " iron " may mean free, uncombined, metallic iron, or iron-matter 

 in some compound. The sense in which the word is employed 

 must be inferred from the context or circumstances. When a 

 chemist speaks, as he sometimes does, colloquially, of " iron " in a 

 drinking water, for example, we know at once that he refers to 

 iron in the form of some compound, for metallic iron does not 

 dissolve in water. 



The word element, then, means one of the simple forms of 

 matter, either free or in combination. 



In formally describing a body or specimen, the chemist always 

 avoids the ambiguity just referred to by naming the components, 

 i.e., the substance or substances it contains. He assumes that 

 the nature and constituents of these substances will be known to 

 anyone hearing or reading the description. If he says the body 

 contains zinc and sulphur, it is understood that the body is a 

 mixture of these simple substances. If it contained these ele- 

 ments in combination, the chemist would report that it was sul- 

 phide of zinc. 



The Common Elements. Thousands of different com- 

 pound substances are known but, when they are decomposed, it is 

 found that the number of different elements contained in them is 

 not great. Dozens of substances contain iron, hundreds contain 

 sulphur, thousands contain oxygen. In fact, by combining a 

 limited number, two, three, or four, of simple substances together, 

 in varying proportions by weight, an almost unlimited number of 

 different compound substances could be produced. 



