ARCHITECTURE OF MOLECULES 221 



figure, or whether the directions of these three should be sup- 

 posed to lie in one plane, the two others being disposed one 

 above and the other below that plane. 



Since the time referred to discoveries have followed one 

 another in rapid succession. Pope and Peachey, and at nearly 

 the same time S. Smiles, succeeded in isolating optically active 

 compounds, of which the atom of sulphur was the directing 

 nucleus. These were followed by compounds of tin, silicon, and 

 phosphorus, which have been shown to be endowed with the 

 same property. And there appears to be no doubt that all the 

 quadrivalent elements to be found in Groups IV and VI of the 

 periodic scheme, as well as the quinquevalent associates or allies 

 of nitrogen and phosphorus, may gather round them groups of 

 other atoms in tridimensional space and can thus act as centres 

 of optical activity. 



Perhaps still more remarkable was the announcement in 1911 

 from the laboratory of Professor Werner of Zurich, whose views 

 were briefly referred to in a former chapter (Electrolysis). From 

 an examination of the complex ammonia compounds, called 

 ammines, produced by several metals, especially platinum, 

 chromium and cobalt, he has been able to prove that metals 

 can act as the central nucleus of stable asymmetric molecules, 

 which may be resolved into optical isomerides exhibiting the 

 optical activity corresponding to that of carbon compounds. 

 The formulae contrived by Werner, it will be remembered, in- 

 volve the assumption of two kinds of valency, namely that which 

 applies to the ionisable salt-forming constituents and that which 

 is concerned in holding together the constituents included in 

 the undissociable zone. 



A large number of substances when passing from the liquid or 

 gaseous to the solid state produce crystals. Thus common salt, 

 the alums, fluorspar, and many other compounds form cubes or 

 regular octahedrons, while quartz is familiar in six-sided prisms, 

 calcspar in rhombohedrons, and green vitriol is at once distin- 

 guished from blue vitriol not only by colour but by the form of the 

 prism assumed by their crystals respectively. Now a crystal is 

 not only characterised by a definite external form, but the 

 material of which it is composed gives evidence of definite 

 internal structure. If for example a rhombohedral crystal of 

 calcite is examined it is found to exhibit double refraction, a 

 phenomenon which is familiar in Iceland spar. And if such 



