252 THE ATOMIC THEOKY. 



cr3^stalline form and in their action upon i)<)larize(l lioht. One acid 

 was dextrorotatory, the other la?vorotatory, while a mixture of the 

 two in equal proportions was neutral to the polarized l)eam, and gave 

 no rotation at all. Their crj^stals exhibited a similar difference in the 

 arrangement of certain planes, one set being right-handed, the other 

 left-handed; and each cr^^stal resembled its isomer like a retiection in 

 a mirror, alike, but reversed. For a long time tliis physical isomerism, 

 as it was called, remained inexplicable, for the rules of valency gave 

 to both molecules the same structure and offered no hint as to the 

 cause of the difference. Structural formuhe, however, said nothing of 

 the arrangement of the atoms in tridimensional space and it Avas soon 

 suspected that the root of the difficulty was here. The mere linking 

 of the atoms with one another could be represented in a single plane, 

 but that was obviously an imperfect symbolism. 



- In 187-t van't Hott' and Le Bel, working independently of each other, 

 suggested a solution of the problem. One simple assumption was 

 enough; merel}^ that the quadrivalent carbon atom was essentially a 

 tetrahedron or, more precisely, that its four units of chemical attrac- 

 tion were exerted from a common center in the direction of four tetra- 

 hedral angles. Atoms of that kind could he built up into structures in 

 which right-handedness and left-handedness of arrangement appeared, 

 provided onl}' that each one was united with four other atoms or 

 groups ail different in nature. Stereochemistry was born, the anom- 

 alies vanished, and many new substances showing optical and crystal- 

 line properties analogous to those of tartaric acid were soon prepared. 

 The theory of van't Hoff' 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 tirst to be assumed. It was only a 

 new extension of Dalton's atomic theory, but it has suggested a future 

 dev(dopment of extraordinary signiticance. If we can determine, not 

 mereiy the linking of the atoms, but also their arrangement in space, 

 we should be able, sooner or later, to establish a connection between 

 chemical composition and crystalline form. The architecture of the 

 molecule and the architecture of the crystal must surely in some way 

 be related. But the pro])lem is exceedingh^ complex, and we may have 

 to wait many 3'ears before we I'each its solution. The atomic theory 

 still has room to grow. 



Let us now turn back in time and consider another phase of our 

 subject. In 1815 Prout suggested that the atomic weights of all the 

 elements were even multiples of that of hydrogen. It was only a 

 speculation on the part of Prout, and yet it led to iniportant conse- 

 quences, for it opened a discussion upon the nature of the chemical 

 elements, and it pointed to hydrogen as the primal matter of the uni- 

 verse. Prout's hypothesis, tlun'efore, l)ecanie a subject of controversy. 

 It foinid many supporters and also many antagonists; l)ut, fortunately, 



