ii The Substance of Nature 25 



and the proportion of each ingredient varies in different 

 specimens. In a mixture each ingredient retains all 

 its own properties, and so can readily be recognised and 

 separated. A mixture of sand, salt, and sawdust, for 

 example, could be separated by throwing it into water, in 

 which the sawdust would float, the sand sink, and the salt 

 dissolve. 



42. Compounds. Quartz, felspar, and mica may be 

 examined as closely as the most powerful microscope allows, 

 but no sign of any of them being a mixture will appear. 

 Every one part of quartz is exactly like every other. Quartz, 

 which is also called silica, can be separated into two sub- 

 stances by means of certain processes explained by the 

 science of chemistry. One of these substances is a brown 

 opaque solid called silicon^ the other an invisible odourless 

 gas named oxygen. Silica is not called a mixture but a 

 compound, the distinction of which is that the components 

 lose all their characteristics and unite to form a homo- 

 geneous substance, different in its properties from any of 

 the components. For example, the metal magnesium is a 

 tough lustrous solid ; oxygen is an invisible gas present in 

 the air ; the compound resulting from their union is a soft 

 snow-white powder. The composition of compounds is 

 always exactly the same, the same proportion of each 

 component being always present. Silica is invariably 

 composed of 1 4 parts by mass of silicon and 1 6 of oxygen ; 

 magnesia always contains 24 parts of magnesium and 16 

 of oxygen. 



43. Analysis of Granite. Felspar may be analysed 

 into silica, alumina, lime, and potash, each one of which 

 is in itself a compound ; and Mica can be analysed into 

 silica, alumina, magnesia, potash, water, and iron oxide, 

 all of which are compounds. The ultimate components 

 are termed elements, of which some, such as oxygen and 

 silicon, are classed as non-metals, the others as metals. 

 Thus : 



