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230 Journal of the Asiatic Soc. of Bengal. [N.S., XVII, 1921.] 
St -peehieiias Prof. Singh, who has himself contributed certain 
observations on the subject, comes to the conclusion that 
neither Frankland’s theory, nor what he terms “‘ Cohen’s rule ”’ 
accords with the facts; but he omits to point out that both 
his own and Frankland’s observations are made with dissolved 
substances in which the solvent may, and frequently does 
modify the rotation, whereas Cohen and his co-workers pur- 
posely avoided the use of any solvent ......... ”” Cohen’s 
observations, it is admitted, were made without the use of any 
solvent, as the substances investigated by him were liquids. 
This could not be done with the compounds investigated. by 
the writer. In failing to apply Cohen’s rule to the cases 
studied by Singh and his co-workers, the remarkable influence of 
solvent on the rotatory power is fully discussed (see p. 233 and 
234) and it is also one of the factors which makes optical activity 
and chemical constitution so difficult to correlate. It would 
ppear from Professor Cohen’s remarks that the rotatory 
power should be determined in the pure state without the us 
of any solvent. Since many of the compounds compared in 
only to be observed in solution, as was pointed out by Pickard 
and Kenyon (T. 1913, 103, 1923). It would therefore appear 
interesting to the writer to see to what extent the regularities 
which have been observed by Cohen in the pure liquid state 
are maintained in solution. 
B. K. 8. 
Patna COLLEGE, 
PATNA. 
