SOME COMPOUNDS OF COMMON ELEMENTS 159 



copper. We add ammonia; if the solution then becomes 

 deep blue, we know that copper is present. 



177. Classes of Compounds. A class of compounds in^ 

 eludes several substances which resemble one another either 

 in their constituents or in their chemical properties, or both. 

 For example, all substances which consist of oxygen and only 



FIG. 78. BENJAMIN SILLIMAN: CHEMIST. 1779-1864 



By his lectures delivered in every part of the country, he contributed, 

 in a large degree, to the promotion of a love of science and to the founda- 

 tion of scientific institutions. DANIEL COIT OILMAN in Leading American 

 Men of Science. * 



one other element are oxides. Besides the oxides, there are 

 three other important classes of common compounds : acids, 

 bases, and salts. 



178. The Two Most Important Oxides. Water, which 

 is a hydrogen oxide, and carbon dioxide are compounds which 

 are absolutely necessary to living things. Naturally, both 

 are provided as a part of the earth itself. Besides being 



