October 2i. Hins.] 



SCIENCE. 



521 



soon suspected that the root of the dit!i- 

 enlty was here. The mere linking of the 

 atoms witli one another could be repre- 

 sented in a single plane, but that was ob- 

 viously an imperfect symbolism. 



In 1874 van't lloff and Le Bel, woi-kiiig 

 independently of each other, .suggested a 

 solution of the problem. One simple as- 

 sumption was enough : merely that the 

 ((uadrivalent carbon atom was essentially 

 a tetrahedron, or. more precisely, that its 

 four units of chemical attraction were ex- 

 erted, from a connnon center, in the direc- 

 tion of four tetrahedral angles. Atoms of 

 that kind could be built up into structures 

 in whieli right-handedness and left-handed- 

 ness of arrangement appealed, provided 

 only that each one was united with four 

 other atoms or groups all dili'erent in na- 

 ture. Stereo-chemistry was born, the 

 anomalies vanished, and many new sub- 

 stances showing optical and crystalline 

 properties analogous to those of tartaric 

 acid were soon prepared. The theory of 

 van't Iloft" and Le Bel was fertile, and 

 therefore it was justified; it interpreted 

 another set of phenomena, but, in order to 

 do so, something like atomic form had first 

 to be assumed. It was only a new extension 

 of Dalton's atomic theory, but it has sug- 

 gested a future development of extraoi-- 

 dinary significance. If we can determine, 

 not merely the linking of the atoms, but 

 also their arrangement in space, we should 

 be able, sooner or later, to establish a con- 

 nection between chemical composition and 

 cry.stalline form. The architecture of the 

 molecule and the architecture of the crystal 

 must surely, in some way. be related. But 

 the problem is exceedingly complex, and 

 we may have to wait many year.s before 

 we reach its solution. The atomic theory 

 still has room to grow. 



Let us now turn back in time, and con- 

 sider another phase of our subject. In 



ISl.j Prout suggested that the atomic 

 weights of all the elements were even mul- 

 tiples of that of hydrogen. It was only 

 a speculation on the part of Prout, and yet 

 it led to important consequences, for it 

 opened a discussion iipon the nature of the 

 chemical elements, and it pointed to hydro- 

 gen as the prinud matter of the universe. 

 Prout 's hypothesis, therefore, became a 

 subject of eontrovei'sy ; it found many sup- 

 porters and also many antagonists ; but, 

 fortunately, one aspect of it was capable 

 of experimental investigation. Some of 

 the most exact and elaborate determination.-* 

 of atomic weight have been nmde with the 

 direct purpo.se of testing the truth or 

 falsity of Prout 's speculation, and science 

 thereby has been notablj' enriched. The 

 marvelous researches of Stas, for instance, 

 had this specific object in view. The ver- 

 dict was finally unfavorable to Prout; at 

 least, the best measurements fail to sup- 

 j)ort his idea ; but it still has advocates who 

 believe that the experimental data are viti- 

 ated by unknown errors and that future 

 investigations will reverse the decision. In 

 science there is no court of last appeal. 



Prout 's hypothesis, then, stimulated the 

 determination of atomic Aveights, and so 

 helped lis to a more accurate knowledge of 

 them. It also led to a search for other re- 

 lations between the.se constants, and thus 

 paved the way for important di.seoveries. 

 Dcibereiner, Kremers, Dumas, Pettenkofer, 

 Cooke and many other chemists publi.shed 

 memoirs upon this theme, but not one of 

 them was general or conclusive.* Groups 

 of elements were compared and relations 

 were brought to light, but an exhaustive 

 study of the question was hardly possible 

 until after Cannizzaro had revised the 



* .■\ very full account of these attempts i.s giveii 

 ill Veiiiible's liook, ' The Development of the Peri- 

 odic Law.' Published at-Kaston, Pennsylvania, 

 in 1896. 



