2 54 



NATURE 



[May 8. 1913 



THE SPECTROSCOPE IN ORGANIC 

 CHEMISTRY. 1 



COME WHAT more than half a century ago, 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 fluorescent screen. 



At the time of this discovery, and in the years 

 immediately following it, attention was being directed 

 fo 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 communicated 

 to the Roval Societv in 1S62, he says : — " Having 



which we now possums of the relation between the 

 structure of organic substances and the action of such 

 substances on the ultra-violet rays, but the elaboration 

 of the convenient and elegant methods by which such 

 investigations are now conducted. 



The light derived from an ordinary source of illu- 

 mination, such as an electric lamp, consists of waves 

 of all degrees of refrangibility, and its spectrum shows 

 a continuous band of colour ranging from red to 

 violet. The limits of this visible spectrum lie between 

 the wave-lengths 7600 and 3900. 



If now, instead of the electric light or other 

 ordinary source of illumination, we employ the light 

 emitted bv one of the metals when raised to a high 

 temperature, the spectrum is seen to consist of a 

 series of lines of different colours and intensities lying 

 within the same limits as the visible spectrum. But 

 there are ravs beyond the red end of the spectrum 

 and rays bevond the violet end which excite no sensa- 

 tion of luminosity in the eye. Bv allowing the spec- 

 trum to fall upon a screen which has been coated 

 with a fluorescent substance, such as sulphate of 

 quinine or a salt of uranium, tnese rays are rendered 

 visible for a short distance beyond the violet. But it 

 is onlv when we replace the glass apparatus, with 



Spark spectrum of nickel and i 



The same, after the light has passed through quartz ro m 

 n glassj[o"33 mm. thick. 5. Window glass i"62 mm. thick. 



obtained the long spectrum above-mentioned I could 

 not fail to be interested in the manner in which sub- 

 stances — especially pure, but otherwise imperfectly 

 known organic substances — might behave as to their 

 absorption 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 be identified. 



While Stokes was engaged in these researches, 

 Prof. William Allen Miller was simultaneously at 

 work in the same field, and Stokes left the further 

 development of the subject in his hands. Miller im- 

 proved the method of observation by substituting a 

 photographic plate for the fluorescent screen, but he 

 failed to " trace any special connection between the 

 chemical complexity of a substance and its diactinic 

 power." Struck by this fact, W. N. Hartley — now 

 Sir Walter Hartley — commenced a systematic inves- 

 tigation of the whole subject, and it is to his re- 

 searches, extending over a period of more than thirty 

 vears, that we owe, not only most of the knowledge 



1 From a discourse delivered at the Royal Insl 

 by Dr. J. T. Dobbie, F.R.S. 



NO. 2271, VOL. 91] 



Friday, April 



which we have hitherto been working, by a quartz 

 prism and lenses, and substitute a photographic plate 

 for the eye, that the full extent of the spectrum 

 beyond the violet is revealed. This is the ultra-violet 

 region — the region which Stokes opened up to inves- 

 tigation, and it is with the behaviour of organic 

 substances towards the rays of this part of the spec- 

 trum that we have mainly to do this evening. 



When light is transmitted through a coloured solu- 

 tion certain rays are absorbed, and dark bands corre- 

 sponding to these rays appear in the spectrum. The 

 importance of. these bands as a means of distinguish- 

 ing coloured substances has long been recognised, 

 and, as we have already seen, considerable progress 

 had been made with their study fifty years ago. As 

 the bands in this case are in the visible spectrum, 

 no special means are required for their observation. 



But when we extend this method of investigation to 

 colourless substances we are dealing with phenomena 

 which lie hidden from the unaided eye, and our in- 

 vestigations are necessarily carried out with the help 

 of Dhotography. 



The instrument employed in the study of absorption 

 spectra consists of a spectroscope in which the eve- 

 piece of the telescope is replaced by a camera. The 



