1903.] on Recent Advances in Stereochemistry. 307 



as dextro- and laevo-rotatory. Van't Hoff and Le Bel declared that 

 the molecules of all naturally occurring substances which exhibit 

 optical activity when in the fluid state contain asymmetric carbon 

 atoms. All substances, the molecules of which contain an asym- 

 metric carbon atom must possess enantiomorphous molecular con- 

 figurations — similar to those assigned to the two lactic acids — 

 because they exhibit properties of an enantiomorphous character. 



A very beautiful experiment which the late Sir G. Gabriel Stokes 

 devised and doubtless exhibited in this room may be so modified as 

 to serve for the demonstration of optical activity. Stokes' experi- 

 ment consists in passing a plane polarised beam of light through a 

 tall cylinder containing water which has been rendered very slightly 

 turbid by the addition of a little alcoholic solution of resin ; a 

 spectrum is then seen spread out in the column of liquid, and spread 

 out in a way which is not enantiomorphous — the water possessing 

 no optical activity. The modification of Stokes' experiment consists 

 in replacing the non-enantiomorphous water by some enantiomorphous 

 liquid— conveniently by a 70 per cent, aqueous solution of the dextro- 

 rotatory cane-sugar or by a 50 per cent, solution of the la)vo- 

 rotatory fruit-sugar ; on making this change, it is seen that instead 

 of the spectrum lying in the cylinder * vertically and therefore non- 

 enantiomorphously, it winds spirally or corkscrew-wise round the 

 column of the enantiomorphous liquid. These spirals or helices 

 are clearly enantiomorphous and the two liquids of opposite optical 

 activity give rise in this experiment to oppositely wound spirals — to 

 spirals which are related to each other like the right- and left-handed 

 corkscrews shown in the lantern slide. The opposite sign of the 

 rotatory power exhibited by the cane-sugar and the fruit-sugar 

 solutions is even more clearly shown by rotating the polarising prism 

 in its mount, when the two sj)irals wind in opposite directions. 



Although cases of optical activity are very frequently met with 

 among chemical substances of animal or vegetable origin, it must be 

 noted that no purely laboratory product or substance prepared without 

 the use of enantiomorphous operations or materials is, in the ordinary 

 way, optically active. The reason of this needs but little seeking if 

 the solid models are once more consulted. Starting with a non- 

 enantiomorphous substance is equivalent to starting with a methane 

 derivative of the constitution — 



and replacing one of the two X groups by the group Q, so as to 

 obtain a compound containing an asymmetric carbon atom. Obviously, 



* In performing this experiment it is convenient to use a glass cylinder 

 about 4 feet in length and 2^ inches in diameter, and to throw the polarised 

 beam vertically tlirougb the column of liquid by means of a 45° prism. 



