i6 Clarke, The Atomic Tlieory. 



precisely, that its four units of chemical attraction were 

 exerted, from a common centre, in the direction of four 

 tetrahedral angles. Atoms of that kind could be built up 

 into structures in which righthandedness and lefthanded- 

 ness of arrangement appeared, provided only that each 

 one was united with four other atoms or groups all different 

 in nature. Stereo-chemistry was born, the anomalies 

 vanished, and many new substances showing optical and 

 crystalline 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 first to be assumed. It was only 

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

 suggested a future development of extraordinary signifi- 

 cance. If we can determine, not merely the linking of the 

 atoms, but also their arrangement in space, wc should be 

 able, sooner or later, to establish a connection between 

 chemical composition and crystalline form. The archi- 

 tecture 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 

 years before we reach 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 important consequences, for it 

 opened a discussion upon the nature of the chemical 

 elements, and it pointed to hydrogen as the primal matter 

 of the universe. Prout's hypothesis, therefore, became a 

 subject of controversy ; it found many supporters and also 

 many antagonists ; but, fortunately, one aspect of it was 



