388 Royal Institution : — 



ROYAL INSTITUTION OF GREAT BRITAIN. 



[Continued from p. 306. J 



March 4, 1864. — "On the Discrimination of Organic Bodies by 

 their Optical Properties." By Professor G. G. Stokes, M.A., D.C.L., 

 Sec. R.S. 



The chemist who deals with the chemistry of inorganic substances 

 has ordinarily under his hands bodies endowed with very definite 

 reactions, and possessing great stability, so as to permit of the em- 

 ployment of energetic reagents. Accordingly he may afford to dis- 

 pense with the aids supplied by the optical properties of bodies, 

 though even to him they might be of material assistance. The pro- 

 perties alluded to are such as can be applied to the scrutiny of 

 organic substances ; and therefore the examination of the bright 

 lines in flames and incandescent vapours is not considered. This 

 application of optical observation, though not new in principle (for 

 it was clearly enunciated by Mr. Fox Talbot more than thirty years 

 ago), was hardly followed out in relation to chemistry, and remained 

 almost unknown to chemists until the publication of the researches 

 of Professors Bunsen and Kirchhoff, in consequence of which it has 

 now become universal. 



But while the chemist who attends to inorganic compounds may 

 confine himself without much loss to the generally-recognized modes 

 of research, it is to his cost that the organic chemist, especially one 

 who occupies himself with proximate analysis, neglects the immense 

 assistance which in many cases would be afforded him by optical 

 examination of the substances under his hands. It is true that the 

 method is of limited application, for a great number of substances 

 possess no marked optical characters ; but when such substances do 

 present themselves, their optical characters afford facilities for their 

 chemical study of which chemists generally have at present little 

 conception. 



Two distinct objects may be had in view in seeking for such infor- 

 mation as optics can supply relative to the characters of a chemical 

 substance. Among the vast number of substances which chemists 

 have now succeeded in isolating or preparing, and which in many 

 cases have been but little studied, it often becomes a question whether 

 two substances, obtained in different ways, are or are not identical. 

 In such cases an optical comparison of the bodies will either add to 

 the evidence of their identity, the force of the additional evidence 

 being greater or less according as their optical characters are more 

 or less marked, or will establish a difference between substances 

 which might otherwise erroneously have been supposed to be 

 identical. 



The second object is that of enabling us to follow a particular 

 substance through mixtures containing it, and thereby to determine 

 its principal reactions before it has been isolated, or even when there 

 is small hope of being able to isolate it ; and to demonstrate the ex- 

 istence of a common proximate element in mixtures obtained from 

 two different sources. Under this head should be classed the detec-- 



