TRANSACTIONS OP SECTION B. 350 



3: The Ultra-Violet Absorption Spectra of the Vapours of various Organic Slib- 

 stances compared with tie Absorption of these Substances in Solution and 

 in Thin Films. By J. E. Purvis. 



Pyridine, a-picoline, 2 : 6-lutidine, 2 : 4-lutidine, 2:4: 6-trimcthyl pyridine 

 and piperidine. — Pyridine vapour under varying conditions of temperature and 

 pressure shows a considerable number of narrow bands which can be arranged in 

 groups having similar appearances and regular differences in their wave lengths ; 

 a-picoline under similar conditions shows a much less number of bands, but 

 some of them are not unlike those of pyridine. Both these substances also 

 exhibit a strong absorption band in the ultra-violet regions and more refrangible 

 than the narrow bands. The vapours of the two lutidines and of trimethyl 

 pyridine do not show any narrow bands; but each shows a strong absorption 

 band analogous to that in pyridine and a-picoline. The vapour of piperidine 

 shows a number of bands which can be divided into groups and which are unlike 

 those of pyridine. 



Nicotine, Coniinc, and Qninoline. — The vapours of these three substances 

 exhibit none of the series of narrow bands found in the vapour of pyridine. 

 Solutions of nicotine show a band analogous to that of pyridine ; solutions of 

 coniine show no band, a result similar to that of piperidine solutions; and qufno- 

 line has one large band as vapour, and three solution bands, as first observed by 

 Hartley. 



Furan, furaldehyde. thiophen, and pyrrole. — Furan vapour shows some narrow 

 absorption bands ; as does the vapour of furaldehyde, but they are different from 

 those of furan; the vapours of thiophen and pyrrole show a few bands, two of 

 which are comparable with two in the vapours of furan and furaldehyde. Solu- 

 tions of furan, thiophen, and pyrrole show no absorption bands : but solutions of 

 furaldehyde show a strong band in the ultra violet. Thin films of these three 

 substances show no selective absoi-ption. 



Aniline, mono- and di-methyl aniline, mono- and di-ethylaniline, o- and 

 m-toluidine, o-3-xylidine, m-2-xylidine, mesidine, and benzylamine. — Aniline 

 vapour shows a considerable number of bands which can be divided into similar 

 groups ; whilst the vapours of the homologues show none of these narrow bands. 

 In the solutions, the single band of aniline is considerably reduced in persistency 

 when the hydrogen of the amino-group of aniline is replaced by alkyl groups; and 

 when the hydrogen of the nucleus is replaced by alkyl groups, the band becomes 

 large and more persistent. 



Chloro- and bromo-benzenes. — Both vapours show a considerable number of 

 absorption bands, which have general relationships amongst themselves, both in 

 structure and in differences of wave lengths. Each group of bands in the bromo- 

 benzene vapour is shifted more towards the red end of the spectrum than the 

 corresponding groups of the chloro-benzenes. Solutions of the substances show 

 seven wide diffuse bands which are comparable in appearance, and only differ in 

 position dependent upon differences in the molecular weights. And very thin 

 films exhibit seven wide diffuse bands comparable with the solution bands : they 

 differ in position according to differences of molecular weights. 



The author is engaged in a comparative study of the o- and jn-dichloro- and 

 dibromobenzenes, and of o- and m-chloro and bromotoluenes. The results show 

 that the vapours of these substances have a number of narrow bands, but fewer 

 than those observed in the chloro- and bromobenzenes : and that their solutions 

 and thin films show none of these narrow bands, but that they are replaced by 

 several wide diffuse bands which differ chiefly in their position dependent upon 

 the molecular weights. The orientation in the o- and wj-compounds is also a 

 factor both in the vapours, in the solutions, and in the thin films. 



These results may be discussed from a consideration of the movements of the 

 atoms of the molecules being influenced by their nature, weight, type, and orienta- 

 tion. The extent of the free path as well as the impacts of symmetrically and 

 unsymmetrically oriented molecules, and possibly the influence of the radiant 

 energy of the source of light, are also to be considered. The vapour molecules 

 have greater freedom of movement, and a considerable number oi bands are pro- 

 duced"; unless, as in the case of the pyridine and aniline derivatives, the unsym- 

 metrical orientation of the atoms of the molecules implies a dislocation in the 

 vibrations so that the rhythmical oscillation is destroyed. In solution the solvent 



