130 Introduction to the Study of Science 



solid except at extremely low temperatures. The candle does 

 not resemble either of its constituents. 



Compare with this the composition of water. Water in the 

 electrolysis experiment was found to be composed of two 

 invisible gases, and it certainly does not resemble them as they 

 exist separately or when mixed. But water is not a mixture 

 such as one may make of sugar and sawdust, or sand and salt. 

 Water is a compound. A compound involves change in the 

 structure of the substances combined to produce it. Hy- 

 drogen and oxygen are changed in their structures in combustion, 

 and so form water. Water is changed in structure when 

 electrical energy, the equivalent of a certain amount of heat, 

 is applied to decompose it into its elements. 



Take now the case of the candle. Heat melts and vaporizes 

 its constituents which, when they unite in burning with oxygen, 

 form new compounds. By the same line of reasoning, one 

 may suppose that hydrogen and carbon were changed in their 

 structures when they were combined to form such compounds 

 as petroleum and its derived product, paraffin. The candle 

 is therefore a typical compound and in no way resembles in 

 structure its constituents as they are when existing separately. 



Compound and simple .substances. Compound substances 

 can be analyzed into two or more substances. Such compounds 

 as carbon dioxid, wood, coal, water, petroleum, sugar, and 

 starch are examples. Just as the paraffin of the candle is de- 

 composed into its constituent substances by burning, so other 

 compounds by suitable treatment may be decomposed into 

 their constituents. But some substances which unite to form 

 these compound substances, or are liberated from them by 

 decomposition, show themselves incapable of further decom- 

 position under any kind of treatment. Such substances are 

 called simple substances or elements to distinguish them from 

 compound substances. Of simple substances or elements, 

 among which are hydrogen, oxygen, nitrogen, and carbon, with 

 which you have become familiar, there are a few more than 



