130 REPORT — 1 882. 



the effect is only about twice as large on the bine band. We must refer 

 to a note by Messrs. Lawrance Smith and Lecoq de Boisbaudran ' for a 

 description of the spectrum of the nitrate of didymium and its changes on 

 addition of nitric acid. 



Some interesting cases of this shifting of bands in different compounds 

 of the same body have been found by Professor Russell,^ who has sub- 

 jected the cobalt salts to a very careful and most instructive examina- 

 tion. The anhydrous bromide of cobalt, for instance, was found to give 

 an absorption spectrum strongly similar to that of the chloride, but there 

 is a general displacement of all the bauds towards the red corresponding 

 to the increased atomic weight of bromine. The effect on the most re- 

 frano-ible band is stronger tlian that on the other two bands, which is 

 contrary to what Bunsen has observed in the case of didymium acetate. 



Captain Abney and Lieutenant- Colonel Testing's paper* ' On the 

 Influence of the Atomic Grouping in the Molecules of Organic Bodies on 

 their Absorption in the Infra-red Region of the Spectrum,' contains an 

 account of investigations undertaken to throw light on the effect of 

 chemical combination on molecular vibrations. The importance of the 

 results which they have obtained will justify a few verbal quotations. 

 They distinguish a general absorption from the least refrangible end, and 

 special absorptions which may consist of lines or bands. 



' Reo-ardino- the general absoi'ption we have nothing very noteworthy 

 to remar-k, beyond the fact that, as a rule, in the hydrocarbons of the 

 same series those of heavier molecular constitution seem to have less than 

 those of lighter.' 



This effect agrees with the observation made by Professors Hartley 

 and Huntington in the ultra-violet in so far as a general shifting of the 

 absorption towards the red seems to take place as the number of carbon 

 atoms is increased. Such a shifting would increase the general absorption 

 in the ultra-violet, as observed by Professors Hartley and Huntington, 

 and decrease it in the infra-red, as observed by Captain Abney and Colonel 

 Festino-. Turning their attention next to the sharply defined lines, our 

 author's, by means of a series of systematic experiments, come to the 

 conclusion that these must be due to the hydrogen atoms in the molecule. 

 ' A crucial test was to observe spectra containing hydrogen and 

 chlorine, hydrogen and oxygen, and hydrogen and nitrogen. 



' We therefore tried hydrochloric acid and obtained a spectrum con- 

 taining some few lines. Water gave lines, together with bands, two lines 

 beino- coincident with those in the specti'um ot' hydrochloric acid. 



'"in ammonia, nitric acid, and sulphuric acid we also obtained sharply- 

 marked lines, coincidences in the different spectra being observed, and 

 nearly eveiy line mapped found its analogue in the chloroform spectrum, 

 and usually in that of ethyl iodide. Benzene again gave a spectrum 

 consisting principally of lines, and these were coincident with some lines 

 also to be found in chloroform. It seems then that the hydrogen, which 

 is common to all these different compounds, must be the cause of the 

 linear spectrum. In what manner the hydrogen annihilates the waves of 

 radiation at these particular points is a question which is at present, at 

 all events, an open one, but that the linear absorptions, common to the 

 hydrocarbons and to those bodies in which hydrogen is in combination 



» C B. Ixxxviii. p. 1167 (1879). '^ Proc. Roy. Soo. xxxii. p. 258 (1881). 



« Phil. Trans, p. 887 (1881, iii.). 



