424 Wright and Kreider — Relation between 



Method of determining the resultant optical activity of a crop of 



crystals. 



There is one point in the determination of the resultant 

 optical activity of crops of crystals which does not appear to 

 have been fully appreciated heretofore, which we would 

 specially emphasize. It has been observed* that not all crys- 

 tals which show a dextro or laevo rotation are necessarily pure 

 forms. At times the twinning is almost if not entirely imper- 

 ceptible, so that a crystal which may appear homogeneous and 

 as a whole rotates the plane of polarization to the right or to 

 the left, is found not to have the proper specific rotation, 

 showing that in fact it is a combination of both forms with a 

 preponderance of one or the other. It is evident, therefore, 

 that in the determination of the excess of one or the other of 

 the enantiomorphic forms in a crop of crystals, whether the 

 basis of comparison be the number of crystals or their weight, 

 an error might result. 



Among the many crops of crystals prepared during this 

 investigation, a number of crystals of this kind appeared, 

 some of which were more or less conspicuous for irregularities 

 in structure and yet to all appearances were decidedly more 

 like a single crystal than an aggregate, but the magnitude of 

 their rotation was not commensurate with their thickness. 



Moreover, very often the crystals are small and intergrown 

 to such an extent as to make an accurate investigation of their 

 rotation very tedious and difficult, if not impossible. 



It is evident then, from both of these considerations, that 

 greater accuracy and economy of time would result from a 

 direct determination of the resultant rotation of the whole 

 crop. This may be accomplished by the elegant method 

 developed by Landolt,f founded upon Christiansen's experi- 

 ments,;); according to which the finely powdered crystals are 

 suspended in a liquid of the same refractive index which has 

 no solvent effect upon them. This method, with some slight 

 modifications which we found desirable, was as follows. 



The crystals were finely powdered in an agate mortar, and in 

 order to secure greater uniformity in size and thoroughness 

 of mixture, passed through a very fine sieve. Having the 

 crystals hot at the start greatly facilitates the attrition as well 

 as the sifting. A mixture of absolute alcohol and carbon 

 disulphide in the proportion of one part of the former to two 

 of the latter furnishes a liquid of the same refractive index in 

 which the crystals are entirely insoluble. Commercial " abso- 



* Marbach, Pogg. Ann., xci, 486 ; Landolt, Ber., xxix, 2, 2412. 

 f Ber., xxix, 2, 2404. 

 X Wied. Ann., xxiii, 298. 



