16 SMITH'S INTERMEDIATE CHEMISTRY 



The gas (later to be named oxygen) was made by Bayen from 

 mercuric oxide, a bright red, rather heavy powder. When 

 the oxide is heated, (Fig. 11), we find that 

 a gas is given off. This gas is easily shown 

 to be different from air, since a glowing splin- 

 ter of wood is instantly relighted on being im- 

 mersed in it. The gas is pure oxygen. During 

 the heating, we notice also that a metallic 

 coating appears on the sides of the tube, in the 

 form of a sort of mirror. Apparently the vapor 

 of some metal is coming off with the oxygen and 

 condensing on the cool parts of the tube. As this shining 

 substance accumulates it takes the form of glolbules, which may be 

 scraped together. It is, in fact, the metal mercury, or quicksilver. 

 If the heating continues long enough, the whole of the red powder 

 eventually disappears, and is converted into these two products. 



Second Variety of Chemical Change: Decomposition. 



Bayen's experiment introduces to us a second, and very common 

 kind of chemical action. The first variety was combination or 

 union (p. 14). The second is called decomposition. It consists 

 in starting with a single substance (here mercuric oxide) and 

 splitting it into two (or more) substances, which differ in proper- 

 ties from the substance taken and from one another. Here, the 

 red powder gave mercury, a liquid metal, and oxygen, a colorless 

 gas. 



Simple and Compound Substances. We have seen that 

 two (or more) substances, like lead and oxygen, can combine to 

 form a compound substance. Are all substances, then, com- 

 pounds? We find that some are not. We have never succeeded 

 in obtaining lead, or oxygen, or iron, or tin, or sulphur by com- 

 bining any two substances. We can decompose mercuric oxide 

 by heat, and we have other ways of decomposing compounds like 

 oxide of tin and ferrous sulphide, but we have never succeeded in 



