LESSONS IN CHEMISTRY. 123 



lated the atomic theory. Avogadro, after a careful study of 

 gases, formulated this theory, that equal volumes of all gases, 

 whether simple or compound, under the same conditions of tem- 

 perature and pressure, contain the same number of molecules. 

 Elements differ in their chemical combinations, as HC1, hydro- 

 chloric acid, H 2 water, NH 3 ammonia, CH 4 marsh gas. The 

 atom of chlorine can hold one atom of hydrogen, the atom 

 of oxygen holds two, the atom of nitrogen three and the 

 atom of carbon four. On examination it is found that a sim- 

 ilar difference occurs among other elements. An element, whose 

 atom can hold only one other atom is called univalent, as 

 hydrogen and chlorine. An element whose atom can hold two 

 others in union is called a bivalent, as oxygen and mercury. An 

 element whose atom can hold three atoms in combination is 

 called trivalent, as nitrogen and antimony, and one that can 

 hold four is called quadrivalent, as carbon and tin. In the forma- 

 tion of salts elements of the same valence replace each other, as 

 in the formation of potassium, nitrate, the univalent potassium 

 replaces the hydrogen in HNO and the compound becomes KNO. 

 In the case of potassium sulphate two atoms of the univalent 

 metal replace the two atoms of hydrogen and H 2 S0 4 becomes 

 K 2 S0 4 . In zinc sulphate one atom of zinc, a bivalent metal, re- 

 places the two atoms of hydrogen and H 2 S0 4 becomes ZnSO 4 

 sulphate of zinc. From the foregoing data we may form a theory 

 for the peculiarities of combination by volume. 



If we suppose our unit volume to contain 1,000 molecules then, 

 according to the law of Avogadro, one volume of hydrogen con- 

 taining 1,000 molecules unites with one volume of chlorine con- 

 taining 1,000 molecules forming two volumes of hydrochloric-acid 

 gas containing 2,000 molecules. But each molecule of the hydro- 

 chloric acid contains two atoms, and in the two volumes there 

 must be 4,000 atoms. From this we judge that the molecules of 

 hydrogen and chlorine are each made up of two atoms. In the 

 case of water we have two volumes of hydrogen containing 2, 000 

 molecules or 4,000 atoms, combining with one volume of oxygen 

 containing 1,000 molecules or two thousand atoms, forming 



