A HUNDRED YEARS OF CHEMISTRY. 679 



the atomic weights in their aspect of combining numbers. They 

 were j)rimarily of nse as factors in chemical calculations, and chem- 

 ists naturally sought for their simplest expressions, with little re- 

 gard to theoretical considerations. The laws of Avogadro, of Du- 

 long and Petit, had, indeed, been announced, but the adjustment 

 of the atomic weights to meet their requirements was long neg- 

 lected. The importance of the adjustment was not realized, for 

 it was obscured by the prevailing dualistic theory, but without it 

 the deeper general relations of the atoms could not appear. Ac- 

 cordingly, a system of chemical formulae grew up Avhich was based 

 upon a deceptive apparent simplicity of ratios, and by which the 

 theory of valence could not be even suggested. The old formula 

 for Avater, HO, expressed only its composition by weight, ignoring 

 its composition by volume; it failed, therefore, to accord with 

 Avogadro's law or to give the slightest hint as to the relations which 

 are now covered by the conception of chemical structure. A part 

 of the existing knowledge was accurately symbolized, but the larger 

 part was ignored, a state of affairs which could not last, although 

 the change came about but slowly. 



The incentive to reform came from two sources. Physics, in 

 the kinetic theory of gases, gave a new demonstration of the truth 

 of Avogadro's law, and led chemists to realize more clearly than 

 before the distinction between atoms and molecules. Soon it was 

 seen that the molecule was the smallest particle of matter which 

 could exist as such, while the atom was the smallest particle which 

 could take part in any chemical change. The metaphysical atom 

 was really the modern molecule; the chemical atom was a new 

 conception, due to the discoveries of chemistry alone. This dis- 

 tinction was found to hold good even for elementary bodies, and 

 it became evident that free hydrogen or oxygen must contain two 

 atoms to the molecule, while phosphorus and arsenic contained 

 four. "With mercury the atom and the molecule are identical, but 

 in most cases the greater complexity exists, and the elements as 

 we see them are compounds of like atoms with each other. That 

 hydrogen can unite with hydrogen, oxygen with oxygen, carbon 

 with carbon, is a conception to which the early chemists never 

 attained, but which is a necessary consequence of Avogadro's law 

 in its application to observed phenomena. 



The second impulse toward change originated in the study of 

 organic compounds, and gained its force from the struggle between 

 contending theories. The advocates of each theory sought for 

 evidence in its favor, and so innumerable discoveries were made, 

 compound radicles were recognized in great numbers, and the mass 

 of data became so overwhelming that for a while chaos reigned. 



