236 Rotation of the Plane of Polarisation of Light. [June 20, 



The measure of the molecular rotation which is usually employed 

 in chemical investigations is 



(M7//)g U bstance / (M<y//>) water, 



where M is the molecular weight. Although the authors postpone a 

 detailed discussion of the validity of this expression, they show that 

 for carbon bisulphide, at any rate, its value changes with the tem- 

 perature, and hence the conclusions obtained by its use regarding 

 questions of chemical constitution, especially of tautomerism, are 

 affected on this account. 



They also point out that the above expression involves the pro- 

 perties of water. The only justification for the use of water in 

 relative observations is the elimination of variations in the strength 

 of the magnetic field in which the observations are made. If the 

 temperature of observation is always the same, this can readily be 

 done. If, on the other hand, the temperature varies, it is essential to 

 know how the rotation of water alters with the temperature. In the 

 past this alteration was unknown, and the arbitrary measure of the 

 molecular rotation above referred to has come into use. Since an 

 expression for the temperature variation has now been obtained it is 

 to be hoped that observers will employ a measure of the molecular 

 rotation which does not involve the properties of water. Indeed, 

 other considerations make such a measure all the more desirable. 

 Up till now the authors have made observations on eight liquids, 

 besides water and carbon bisulphide, and in all cases except that 

 of water the relation between rotation and temperature is linear, 

 and the quotient, rotation divided by density, diminishes as the tem- 

 perature rises. It is highly probable, therefore, that as regards 

 magnetic rotation, as in the case of so many other properties, the 



