264 INORGANIC CHEMISTRY 



opened. Much may be done by treating silicates with appropriate sol- 

 vents, acid or alkaline, which differentiate between uncombined and 

 combined silica, and this in some cases, by replacement of one metal 

 by another, gives a clue to constitution. The complexity of the mole- 

 cules of inorganic compounds, which are usually solid, forms another 

 bar to investigation. It is clear that sulphuric acid, to choose a com- 

 mon instance, possesses a very complicated molecule; and the fused 

 nitrates of sodium and potassium are not correctly represented by the 

 simple formulae NaNO 3 and KN0 3 . Any theory of the structure of 

 their derivatives must take such facts into consideration; but we ap- 

 pear to be getting nearer the elucidation of the molecular weights of 

 solids. Again, the complexity of solutions of the most common salts is 

 maintained by many investigators; for example, a solution of cobalt 

 chloride, while it undoubtedly contains among other constituents 

 simple molecules of CoCl 2 , also consists of ions of a complex character, 

 such as(CoC! 4 ) " . And what holds for cobalt chloride also undoubtedly 

 holds for many similar compounds. 



In determining the constitution of the compounds of carbon, stereo- 

 chemistry has played a great part. The ordinary structural formulae 

 are now universally acknowledged to be only pictorial, if, indeed, that 

 word is legitimate; perhaps it would be better to say that they are 

 distorted attempts at pictures, the drawing of which is entirely free 

 from all rules of perspective. But these formula? may in almost every 

 case be made nearly true pictures of the configuration of the mole- 

 cules. The benzene formula, to choose an instance which is by no 

 means the simplest, has been shown by Collie to be imitated by a 

 model which represents in an unstrained manner the behavior of that 

 body on treatment with reagents. But in the domain of inorganic 

 chemistry, little progress has been made. Some ingenious ideas of the 

 geologist Sollas on this problem have hardly received the attention 

 which they deserve; perhaps they may have been regarded as too 

 speculative. On the other hand, Le Bel's and Pope's proof of the 

 stereo-isomerism of certain compounds of nitrogen; Pope's demon- 

 stration of the tetrahedral structure of the alkyl derivatives of tin; 

 and Smiles's syntheses of stereo-isomeric sulphur compounds give us 

 the hope that further investigation will lead to the classification of 

 many other elements from this point of view. Indeed, the field is 

 almost virgin soil; but it is well worth while cultivating. There is no 

 doubt that the investigation of other organo-metallic compounds will 

 result in the discovery of stereo-isomerides; yet the methods of inves- 

 tigation capable of separating such constituents have in most cases 

 still to be discovered. 



The number of chemical isomerides among inorganic compounds is 

 a restricted one. Werner has done much to elucidate this subject in 

 the case of complex ammonia derivatives of metals and their salts; 



