228 CHEMISTRY 



foreshadowings of law, now fell into place together, and one simple 

 generalization occupied the field. The atomic weights became more 

 than ever the fundamental constants of chemistry, and all the pro- 

 perties of the elements were seen to be periodic functions of these 

 quantities. In Mendelejeff's table stress was laid upon valence and 

 the form of compounds which each element could yield; in Lothar 

 Meyer's curves the physical relations were emphasized, and so each 

 statement reinforced the other. Newlands, it is true, had partially 

 anticipated Mendelejeff, but his law of octaves fell just short of com- 

 pleteness. 



At first, the periodic classification attracted comparatively little 

 attention, and its general acceptance might have been slow had it 

 not been for certain prophecies. In Mendelejeff's table there were 

 many gaps; these were attributed to the existence of elements as 

 yet unknown, and for three of them the author ventured upon pre- 

 dictions. Each element must have a certain atomic weight, a pre- 

 scribed density and melting-point, and should form compounds of 

 a stated character. In due time the three unknown elements were 

 actually found, and gallium, scandium, and germanium confirmed 

 all of Mendelejeff's anticipations. The importance of the classifica- 

 tion was thus established, and the periodic law became one of the 

 foundation stones of modern chemistry. The conception of valence 

 as a property of the atom acquired a broader significance; in cases 

 that had been doubtful its magnitude could be determined, and with 

 its aid the chaos of inorganic chemistry began to exhibit signs of 

 something like order. The deficiencies of the periodic system I need 

 not mention here, for this is no time for details; neither shall I dis- 

 cuss the obvious difficulties which arise when we seek to apply the 

 doctrine of valence to inorganic compounds; only the larger verities 

 concern us now. In the broadest sense the periodic classification is 

 sound; the principle of valence is general, and the obstacles which 

 now appear will doubtless be overcome by future investigation. 

 That the greatest generalization has been reached, we cannot assume; 

 but so far as we have gone we stand on solid ground, and can continue 

 our explorations in safety. 



Up to a certain point organic compounds had been successfully 

 interpreted in terms of valence. Isomerism was explained, and the 

 existence of unknown isomers could be predicted; different atoms 

 were assignable to different positions within the molecule, as in the 

 case of the four hydrogen atoms of acetic acid, one fixed and three 

 replaceable; but after all this had been done there were still some 

 difficulties outstanding. Isomers existed whose chemical structure 

 seemed to be the same, and for their interpretation an extension of 

 chemical theory was needed. This want was supplied by van't Hoff 

 and Lebel, who almost simultaneously pointed out the consequences 



