1 



THE SPACE RELATIONS OF ATOMS. 



( Cone hided. ) 



'HE coincidence of optical activity with molecular 

 dissymmetry having been established, as regards, 

 at least, the great majority of compounds of the type 

 CR'R'R R'\ the question arose : Can the degree of asym- 

 metry be measured by the degree of optical activity ? 



But in view of the fact that the change of rotation 

 resulting from a change of conditions (temperature, wave- 

 length of light, solvent, concentration) is not the same 

 for different substances, the preliminary question arose : 

 Under what conditions can comparable numbers be ob- 

 tained for the optical activity of various substances ? 



The most important variations are those produced by 

 the nature and proportion of the solvent ; in the attempt to 

 avoid these variations by examining the pure substance, a 

 greater difficulty arises from the action which the molecules 

 of the substance itself are then free to exert on one another. 

 This compels a resort to dilute solutions, and the difficulties 

 here encountered are greatly diminished if we consider the 

 dissolved substances as divided into two classes, electrolytes 

 and non-electrolytes — a division forced upon us by the 

 application of the theory of electrolytic dissociation. 



As regards electrolytes the theory teaches that different 

 salts of the same active acid or base possess in dilute solu- 

 tion the same molecular rotation, because they contain ap- 

 proximately the same number of active ions ; this identity 

 of rotation is the law experimentally established by Oude- 

 manns and Landolt. 



For non-electrolytes the results obtained lack precision, 

 because the molecular weight of the active body, and the 

 possible action of the solvent, have in many cases still to be 

 determined. 



The effect of temperature and of wave-length is of minor 

 importance, and, by using the same solvent in the same pro- 

 portion under similar conditions, comparable results may be 



