112 COMBINING PROPORTIONS. 



the constituents being always present in the proportion of two 

 or more equivalents. 



The combining proportions of compound bodies depend 

 entirely therefore upon those of their constituents, or upon the 

 equivalents of the elementary bodies. The mode of determining 

 these fundamental equivalents generally consists, as may be 

 anticipated, in ascertaining the quantity of any element which 

 exists united with 100 parts of oxygen in the protoxide of that 

 element, which quantity is viewed as a single equivalent. Thus of 

 hydrogen and lead, the protoxides are water and litharge, in which 

 respectively 100 oxygen are associated with 12.5 hydrogen and 

 1294 lead, which numbers are therefore single equivalents of these 

 elementary substances, But the difficulty still remains to know 

 what is a protoxide ; for the rule is not followed in all cases to 

 consider that oxide of an element as the protoxide which contains 

 the least proportion of oxygen. When only one oxide is known, 

 it is presumed to be a protoxide and composed of single equiva- 

 lents, unless it corresponds in properties with a higlier degree 

 of oxidation of some other element ; and of several oxides of 

 the same element that containing least oxygen is viewed as the 

 protoxide, unless a higher oxide has better claims to be con- 

 sidered as such. Hence magnesia and oxide of zinc being the 

 only oxides of magnesium and zinc known are protoxides ; 

 and water, litharge, potash, soda, lime, and protoxide of iron, 

 which are all the lowest oxides of different metals, are admitted 

 without objection to be protoxides, and become standards of 

 comparison for this class of bodies ; while alumina, the only oxide 

 of aluminum, differing entirely from the protoxide of iron but 

 closely resembling the peroxide of that metal, is considered a 

 peroxide of similar constitution, or to contain three equivalents 

 of oxygen and two of metal. Now in alumina 300 oxygen, or 

 three equivalents, are united with 342 aluminum, one half of 

 which number, or 171, is therefore the equivalent of aluminum. 

 The true protoxide of aluminum, if it is capable of existing, 

 still remains to be discovered. The first degree of oxidation of 

 chromium, or the green oxide, is likewise a peroxide and not a 

 protoxide, being analogous to alumina and the peroxide of iron. 

 On the other hand the second degree of oxidation of copper, or 

 the black oxide, and not the first degree of oxidation of that 

 metal, must be viewed as the protoxide, or as composed of 

 single equivalents, from its correspondence with the protoxide 



