A HISTORY OF SCIENCE 



vapor, the simplest mathematics show, in the light of 

 Avogadro's law, not only that each molecule of water 

 must contain two hydrogen atoms (a point previously 

 in dispute), but that the original molecules of hydrogen 

 and oxygen must 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 each 

 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 con- 

 ditions which need not detain us here), then each oxy- 

 gen 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 



