THE CENTURY'S PROGRESS IN CHEM1STRV 



are classifications rather than explanations of chemical 

 unions. But at least they served an important purpose 

 in giving definiteness to the idea of a molecular struct- 

 ure built of atoms as the basis of all substances. Now 

 at last the word molecule came to have a distinct mean- 

 ing, as distinct from "atom," in the minds of -the gener- 

 ality of chemists, as it had had for Avogadro a third of 

 a century before. Avogadro's hypothesis that there are 

 equal numbers of these molecules in equal volumes of 

 gases, under fixed conditions, was revived by Gerhard t, 

 and a little later, under the championship of Cannizzaro, 

 was exalted to the plane of a fixed law. Thenceforth 

 the conception of the molecule was to be as dominant a 

 thought in chemistry as the idea of the atom had be- 

 come in a previous epoch. 



Of course the atom itself was in no sense displaced, 

 but Avogadro's law soon made it plain that the atom had 

 often usurped territory that did not really belong to it. 

 In many cases the chemists had supposed themselves 

 dealing with atoms as units where the true unit was the 

 molecule. In the case of elementary gases, such as hy- 

 drogen and oxygen, for example, the law of equal num- 

 bers of molecules in equal spaces made it clear that the 

 atoms do not exist isolated, as had been supposed. Since 

 two volumes of hydrogen unite with one volume of oxy- 

 gen to form two volumes of water vapor, the simplest 

 mathematics shows, in the light of Avogadro's law, not 

 only that each molecule of water must contain two hy- 

 drogen atoms (a point previously in dispute), but that 

 the original molecules of hydrogen and oxygen must 



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