LIFE, CARBON'S OUTSTANDING PROPERTY 57 



particles of sulfur lying next to the grey iron filings. With 

 the help of a magnet, many of the iron filings can be re- 

 moved from the mixture. Or the sulfur can be extracted 

 by means of a solvent like carbon bisulfide, which will dis- 

 solve the sulfur, leaving the iron filings behind. However, 

 if the mixture is heated in one place, it at once flares up to 

 red heat that rapidly spreads over the whole mixture like 

 an explosion. Once this has happened and the mass has 

 cooled down again, it is no longer a simple mixture of iron 

 and sulfur particles, but a chemical compound, iron sul- 

 fide. The individual particles of iron have disappeared, 

 and the sulfur cannot be extracted by any solvent, because 

 iron and sulfur have united to form a new substance. 



Such a formation of new material through the interaction 

 of two completely different substances is called a chemical 

 reaction. In the case just discussed it is a union of the two 

 elements iron and sulfur. Elements we call them, since 

 they cannot be decomposed by simple means. A union of 

 two elements invariably occurs in definite proportions by 

 weight. In our case each gram of iron filings combines 

 with f of a gram of sulfur powder. This means that we 

 have to mix seven parts of iron filings with four parts of 

 sulfur powder if we wish to obtain iron sulfide in pure form 

 free from uncombined sulfur or iron. If we add more sul- 

 fur some of it can be extracted after the reaction with a 

 suitable solvent. If we add more iron, it will not combine. 



Another example is the union of hydrogen and oxygen, 

 both gases, uniting with each other with explosive violence 

 if mixed and then touched with a spark. Or if hydrogen 

 gas is allowed to stream into oxygen, or into air, it can be 

 ignited and will burn with a very hot flame. In this flame 

 a union of hydrogen and oxygen is formed which is water 

 in the form of vapor. The union invariably occurs in the 

 proportion of one part by weight of hydrogen to eight parts 

 of oxygen. 



