THE STORY OF NINETEENTH-CENTURY SCIENCE 



gen is raonovalent, 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 chemist, and called hydroxyl) ; 

 H O H (H 2 0, or water), and H O O H (H 2 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 unstable union, since the avidity of 

 an atom for its own kind is relatively weak. Thus the 

 well-known properties of hydrogen peroxide are ex- 

 plained, its easy decomposition, 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 

 the trio, linked in this close bond, have no tendency to 

 reach out for any other companion, nor, indeed, any 

 power to hold another should it thrust itself upon them. 

 They form a "stable" compound, which under all ordi- 

 nary circumstances will retain its identity as a molecule 

 of water, even though the physical mass of which it is 

 a part changes its condition from a solid to a gas from 

 ice to vapor. 



But a consideration of this condition of stable equi- 

 librium in the molecule at once suggests a new question : 



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