THE SPACE RELATIONS OF ATOMS. 133 



to take place always at the same phase of the oscillation of 

 an atom so as not to interfere with the periodic nature of 

 the oscillations, and Van't Hoff admits that " he who pre- 

 fers to assume atomic motion may consider the points, de- 

 scribed as stationary, as positions about which the motion, 

 probably periodical, takes place ". 



This has from the first been the attitude of Van't Hoff, 

 though the doctrine of universal dissociation appears only 

 in the latest edition of his book (1894) ; and it was natural 

 that in its inception stereochemistry should take this trend, 

 seeino- that its first task was to account for cases of 

 "abnormal" isomerism, and that it was only later it began 

 to concern itself with "abnormal" reactions, and therefore 

 with the problem of atomic dynamics. 



It was the discovery by Wislicenus in 1869 of a second 

 active lactic acid, which, following on the work of Pasteur 

 already mentioned, forced chemists to see the insufficiency 

 of the ordinary structural formulae, of which there is only 

 one for these two substances, viz., CH 3 .CH.OH. COOH. 



At this time attempts at three-dimensional formulae had 

 already been made ; various chemists had devised models 

 for the purpose of illustrating the binding-power or valence 

 of the various atoms, each binding-unit being represented 

 by a rod attached to the sphere which stood for the atom ; 

 these rods were generally all in one plane, and Kekule was 

 the first (1867) who, in order to represent the power of a 

 carbon atom to combine with another by one, two, or three of 

 its units of valence, directed the rods in this case towards 

 the corners of the tetrahedron described about the carbon- 

 sphere. For a time the influence of Wislicenus' discovery 

 on such speculations was not apparent ; thus in 1873 there 

 was published Gaudin's U architecture ciu monde des atonies 

 devoilant la structure des composes chimiques et leur crystal- 

 logenie} in which the author is guided entirely by certain 

 laws of symmetry similar to those which apply in crystal- 

 lography ; and in 1875 F. W. Clarke 2 in a paper called 

 "Chemistryin Three Dimensions" argued that themonatomic 



1 Paris, Gauthier Villers. 



2 Proc. Am. Ass. Adv. Sci., p. 99, 1875. 



