A HISTORY OF SCIENCE 



lent, etc., were coined to express this most important 

 fact, and the various elements came to be known as 

 monads, diads, triads, etc. Just why different elements 

 should differ thus in valency no one as yet knows ; it is 

 an empirical fact that they do. And once the nature of 

 any element has been determined as regards its valency, 

 a most important insight into the possible behavior of 

 that element has been secured. Thus a considera- 

 tion of the fact that hydrogen is monovalent, while 

 oxygen is divalent, makes it plain that we must expect 

 to find no more than three compounds of these two 

 elements namely, H O (written HO by the chem- 

 ist, and called hydroxyl); H O H (H a O, or water), 

 and H O O H (H a O 2 , or hydrogen peroxide). 

 It will be observed that in the first of these compounds 

 the atom of oxygen stands, so to speak, with one of its 

 hands free, eagerly reaching out, therefore, for another 

 companion, and hence, in the language of chemistry, 

 forming an unstable compound. Again, in the third 

 compound, though all hands are clasped, yet one pair 

 links oxygen with oxygen ; and this also must be an un- 

 stable union, since the avidity of an atom for its own 

 kind is relatively weak. Thus the well-known prop- 

 erties of hydrogen peroxide are explained, its easy de- 

 composition, and the eagerness with which it seizes 

 upon the elements of other compounds. 



But the molecule of water, on the other hand, has its 

 atoms arranged in a state of stable equilibrium, all their 

 affinities being satisfied. Each hydrogen atom has sat- 

 isfied its own affinity by clutching the oxygen atom; 

 and the oxygen atom has both its bonds satisfied by 

 clutching back at the two hydrogen atoms. Therefore 



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