ATOMIC WEIGHTS. 55 



peroxide of iron is composed of 12 parts oxygen to 

 28 of iron. Oxygen combines with manganese 

 in the same ratio, forming a sesquioxide, from 

 the latin word sesqui, one and a half. It also 

 unites with arsenic in the ratio of 12 to 38 of the 

 metal, as in arsenious acid ; and of 20 to 38, as in 

 the composition of arsenic acid. With phos- 

 phorus, it unites in the ratio of 4, 12, and 20, as 

 in hypophosphorous acid, phosphorous, and phos- 

 phoric acids ; which would seem to indicate, that 

 oxygen is capable of combining with the above 

 substances in the ratios of \, 1^, or 2-J- atoms. 



This difficulty has been evaded by supposing 

 that 2 equivalents or atoms of one substance 

 unite with 3, 5, or more atoms $f another that if 

 2 atoms of phosphorus unite with 3, and 5 atoms 

 of oxygen, we obtain the ratios of 1-f- and 2-i- ; or 

 if one atom of iron unite with 1 of oxygen, and 

 another compound be formed of 2 atoms of iron 

 to 3 of oxygen, the latter would be in the propor- 

 tions of 1, and 1-f-, and so of other like cases. 



When 1 atom of oxygen unites with 2 of lead, 

 they form a ternary atom of what is termed a 

 dinoxide, or a suboxide. Dr. Turner lays it down 

 as a general rule, that when a metal forms two 

 oxides, the oxygen of which is in the ratio of 

 I to 1^, the first is usually the protoxide, and the 

 second a compound of two atoms or equivalents 

 of the metal to 3 of oxygen, as in the oxides of 

 iron nickel, and chromium. 



It is highly probable, that a more accurate and 



