PROGRESS IN PHYSICAL CHEMISTRY. 237 



It. is noteworthy that more attention is being paid to the 

 influence of temperature. This effect is most remarkable, 

 as in some cases the rotatory power rapidly increases as 

 the temperature rises. Colson (45) has made experiments 

 to show that this may be due to decomposition of the sub- 

 stances. Le Bel (46) finds, however, that in the case of 

 methyl and ethyl tartrates the change is not due to the 

 presence of acid or alcohol, nor, according to Ramsay's 

 measurements, can it be ascribed to alteration in the mole- 

 cular complexity of the liquid, at any rate above 70 . 



Several observers are now using in place of the old 



measure of the molecular rotation, the molecular deviation 



proposed by Guye, which is the product of the angle of 



rotation for unit length and the cube root of the molecular 



volume. It is questionable how far this is an advantage. 



For although the old measure is not all that could be 



desired, on theoretical grounds the molecular deviation can 



hardly be held to be an improvement. As I have pointed 



out (47), it would only give the effect produced by the same 



number of molecules of different liquids, if the region within 



wmich the ray could be influenced by a molecule were 



proportional to the molecular volume, and for this there is 



no support in fact. Of course in the present state of the 



question the agreement with observation is the best test of 



the worth of either formula. Since there is little to choose 



between them on this score it is to be hoped that observers 



will not disregard the old measure until more light has been 



thrown upon the question. - 



On account of the increased rotation, measurements of 

 optical activity would be much enhanced in accuracy if light 

 of shorter wave-length than D-light could conveniently be 

 •obtained. Landolt (48) has recently described a series of 

 absorption cells which on passing white light through them 

 give beams of different colours. The light thus obtained is 

 not strictly monochromatic, but appears to give the same 

 rotation as light of a definite wave-length. 



Another important practical point is the modification of 

 Lippich's polarising prism (49). Instead of using one 

 auxiliary Nicol he now uses two, the field of view being 



