58 A YEAR IN SCIENCE 



A small amount of mercuric oxide, a light red powder, 

 is placed in a test tube. The tube is then thoroughly 

 heated. If, while the tube is in the flame, a glowing pine 

 splinter is inserted into it, the splinter at once breaks 

 into a flame. Evidently there is something being given off 

 from the highly heated mercuric oxide which causes this 

 change in the splinter. This substance is the gas, oxygen, 

 an element which forms about one-fifth of the air. 



If the tube is allowed to cool, the inside of it will be 

 found to be covered with a thin coating of mercury. 

 Thus mercuric oxide has been separated by means of 

 heat into its elements, oxygen and mercury. This 

 process of separating or decomposing a compound into 

 its elements is known as chemical analysis. 



Chemical affinity. It is evident that in the formation 

 of the chemical compound, iron sulphide, the two 

 elements, iron and sulphur, were in some way com- 

 bined. This combination was brought about by the 

 action of heat. The force which caused these two 

 elements to combine and which held them together 

 when combined is called chemical affinity. 



The attraction, or affinity, which one substance has 

 for another is always the cause of chemical union. 

 Frequently this union does not take place except 

 through the action of some agency, as heat, light, or 

 electricity. \ 



Sometimes these same agencies may overcome this 

 attraction and as a result cause the decomposition of 

 a compound. Mercuric oxide we have learned is com- 



