APPENDIX n. 487 



In indicating that the gradual increase of the power of elements of com- 

 bining with oxygen ia accompanied by a corresponding decrease in their 

 power of combining with hydrogen, the periodic law has shown that there is 

 a limit of oxidation, just as there is a well-known limit to the capacity of 

 elements for combining with hydrogen. A single atom of an element com- 

 bines with at most four atoms of either hydrogen or oxygen ; and while CH 4 

 and SiH 4 represent the highest hydrides, so Bu0 4 and OsO 4 are the highest 

 oxides. We are thus led to recognise types of oxides, just as we have had to 

 recognise types of hydrides. 12 



The periodic law has demonstrated that the maximum extent to which 

 different non-metals enter into combination with oxygen is determined by the 

 extent to which they combine with hydrogen, and that the sum of the number 

 of equivalents of both must be equal to 8. Thus chlorine, which combines 

 with 1 atom or 1 equivalent of hydrogen, cannot fix more than 7 equivalents 

 of oxygen, giving C^Of ; while sulphur, which fixes 2 equivalents of hydrogen, 

 cannot combine with more than 6 equivalents or 3 atoms of oxygen. It thua 

 becomes evident that we cannot recognise as a fundamental property of the 

 elements the atomic valencies deduced from their hydrides ; and that we 

 must modify, to a certain extent, the theory of atomicity if we desire to raise 

 it to the dignity of a general principle capable of affording an insight into the 

 constitution of all compound molecules. In other words, it is only to carbon, 

 which is quadrivalent with regard both to oxygen and hydrogen, that we can 

 apply the theory of constant valency and of bond, by means of which so many 

 still endeavour to explain the structure of compound molecules. But I should 

 go too far if I ventured to explain in detail the conclusions which can be 

 drawn from the above considerations. Still, I think it necessary to dwell 

 upon one particular fact which must be explained from the point of view of 

 the periodic law in order to clear the way to its extension in that particular 

 direction. 



The higher oxides yielding salts the formation of which was foreseen by 

 the periodic system for instance, in the short series beginning with sodium 



Na,0, MgO, A1A, Si0 2 , P 2 5 > S0 3 , CIA. 



must be clearly distinguished from the higher degrees of oxidation which cor- 

 respond to hydrogen peroxide and bear the true character of peroxides. Per- 

 oxides such as Na 3 O a , BaO 2 , and the like have long been known, Similar 



12 Formerly it was supposed that, being a bivalent element, oxygen can enter into any 

 grouping of the atoms, and there was no limit foreseen as to the extent to which it could 

 further enter into combination. We couldnot explain why bivalent sulphur, which forms. 

 compounds such as 



could not also form oxides such 



while other elements, as, for instance, chlorine, form compounds such as 



Cl-O-Q 0-0 K 

 *J 



