CHEMISTRY SINCE TIME OF DALTON 



doctrine of compound radicals, became useful as aids 

 to memory and guides for the analyst, indicating some 

 of the plans of molecular construction, though by no 

 means penetrating the mysteries of chemical affinity. 

 They are classifications rather than explanations of 

 chemical unions. But at least they served an impor- 

 tant purpose in giving definiteness to the idea of a 

 molecular structure built of atoms as the basis of all 

 substances. Now at last the word molecule came to 

 have a distinct meaning, as distinct from "atom," in 

 the minds of the generality of chemists, as it had had 

 for Avogadro a third of a century before. Avogadro's 

 hypothesis that there are equal numbers of these mole- 

 cules 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 become in a previous 

 epoch. 



CHEMICAL AFFINITY 



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 them- 

 s dealing with atoms as units where the true unit 

 was the molecule. In the case of elementary gases, 

 such as hydrogen and oxygen, for example, the law of 

 equal numbers of molecules in equal spaces made it 

 clear that the atoms do not exis.t isolated, as had been 

 >sed. Since two volumes of hydrogen unite with 

 one volume of oxygen to form two volumes of water 



57 



