CHEMICAL CHANGE AND THE METHODS OF STUDYING IT 17 



decomposing the mercury or the oxygen, the iron or the sulphur 

 themselves. Substances which we are not able, by chemical 

 means, to decompose into, or to make by chemical union from, 

 other substances are called simple or elementary substances. 

 The distinction between simple and compound substances was 

 first drawn by Boyle in 1678. Later, and independently, it was 

 stated very clearly by Lavoisier (1789). 



Several substances, regarded in Lavoisier's time as elementary, 

 have since been shown to be compounds. Thus, quicklime was a 

 simple substance until Davy, in 1808, prepared the metal calcium 

 and showed that quicklime was the oxide of this metal. Hence, 

 we do not say that the substances regarded as simple cannot be 

 decomposed, but only that they are substances which we " are not 

 able " (at present) to decompose. 



The phrase " by chemical means " is also important. Although 

 by chemical methods we are not able to effect any decomposition 

 of the elements, yet we cannot regard them as absolutely unalter- 

 able and permanent. The element radium cannot be decomposed 

 by chemical means, but it undergoes continuous and spontaneous 

 " disintegration " into the elements helium and lead (see p. 546). 

 It has recently been discovered by Rutherford that other elements, 

 such as nitrogen . and aluminum, may also be disrupted to give 

 hydrogen, under the action of the tremendous forces of the 

 swiftly-moving particles ejected from radium in the course of 

 this disintegration. Such phenomena, however, do not affect 

 our conception of elements as applied to ordinary chemical reac- 

 tions. 



Elements. The word element is used in two senses. It is 

 applied to the simple substance. Thus we speak of " the element 

 iron," meaning the metal iron. It is applied also to the iron- 

 matter contained in ferrous sulphide or in ferric oxide. The 

 reader should note that it is correct usage to speak of the element 

 iron and the element sulphur in ferrous sulphide, but a chemist 



