THE STORY OF NINETEENTH-CENTURY SCIENCE 



have been composed in each case of two atoms else 

 how could one volume of oxygen supply an atom for 

 every molecule of two volumes of water? 



What, then, does this imply ? Why, that the ele- 

 mentary atom has an avidity for other atoms, a long- 

 ing for companionship, an " affinity " call it what you 

 will which is bound to be satisfied if other atoms are 

 in the neighborhood. Placed solely among atoms of 

 its own kind, the oxygen atom seizes on a fellow oxy- 

 gen atom, and in all their mad dancings these two 

 mates cling together possibly revolving about one an- 

 other in miniature planetary orbits. Precisely the same 

 thing occurs among the hydrogen atoms. But now 

 suppose the various pairs of oxygen atoms come near 

 other pairs of hydrogen atoms (under proper conditions 

 which need not detain us here), then each oxygen atom 

 loses its attachment for its fellow, and flings itself madly 

 into the circuit of one of the hydrogen couplets, and 

 presto ! there are only two molecules for every three 

 there were before, and free oxygen and hydrogen have 

 become water. The whole process, stated in chemical 

 phraseology, is summed up in the statement that under 

 the given conditions the oxygen atoms had a greater 

 affinity for the hydrogen atoms than for one another. 



As chemists studied the actions of various kinds of 

 atoms, in regard to their unions with one another to 

 form molecules, it gradually dawned upon them that 

 not all elements are satisfied with the same number of 

 companions. Some elements ask only one, and refuse 

 to take more ; while others link themselves, when occa- 

 sion offers, with two, three, four, or more. Thus we 

 saw that oxygen forsook a single atom of its own kind 

 and linked itself with two atoms of hydrogen. Clearly, 



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