1913] The Spectroscope in Organic Ciiemistry 703 



WEEKLY EVENING MEETING, 

 Friday, April 4, 1913. 



Alexander Siemens, Esq., M.Inst.C.E., Vice-Presideut, 

 in the Chair. 



James J. Dobbie, Esq., M.A. LL.D. D.Sc. F.R.S. M.R.I. 



The Spectroscope in Organic Chemistry. 



Somewhat more than half a century a,o;o, while engaged, with the 

 assistance of Faraday, in preparing experiments for a Friday evening 

 discourse in this Institution, Stokes observed that the spectrum of 

 the electric light extended to five or six times the length of the 

 visible spectrum when he employed prisms and lenses of quartz 

 instead of glass. This extension occurs at the violet end of the 

 spectrum, and consists of rays of high refrangibility to which the eye 

 is insensitive, but which can be made apparent by means of a fluor- 

 escent screen. 



At the time of this discovery, and in the years immediately follow- 

 ing it, attention was being directed to the absorption of light by 

 coloured solutions, and to the possibility of identifying coloured 

 substances by the number and position of the dark bands in the 

 spectrum of light transmitted through their solutions. Stokes saw 

 that by his discovery of the extension of the spectrum beyond the 

 visible region, this method of investigation might be applied to 

 colourless as well as to coloured substances. In a paper communi- 

 cated to the Royal Society in 1862, he says : — " The mode of absorp- 

 tion of light by colourless solutions as obsei'ved by the prism, affords, 

 in many cases, most valuable characters of particular substances, which, 

 strange to say, though so easily observed, have till very lately been 

 almost wholly neglected by chemists." ..." Having obtained the 

 long spectrum above mentioned I could not fail to be interested in 

 the manner in which substances — especially pure, but otherwise imper- 

 fectly known organic substances — might behave as to their absorp- 

 tion of the rays of high refrangibility." He proceeded, therefore, to 

 study the action of various organic solutions on the ultra-violet rays, 

 and found that the mode of absorption generally was so constant and 

 so characteristic that by this single property many substances could 

 he identified. The sulistances which he examined included the best 

 known vegetal)le alkaloids, all of which, with the exception of mor- 

 phine and codeine, he was able to distinguish from one another by 

 means of their absorption bands. This was a very remarkable antici- 

 pation of the results of later work on the subject, when we remember 



