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XX. On the Influence of the Atomic Grouping in the Molecules of Organic Bodies on 
their Absorption in the Infra-red Region of the Spectrum. 
By Captain Abney, R.E., F.R.S., and Lieut.--Colonel Festing, R.E. 
Received February 5—Read February 10, 1881. 
[Plates 86-88,] 
Introductory. 
The researches on which this paper is founded were commenced in February, 1880, 
but were not sufficiently advanced for any communication to he made regarding them 
during last session. As an article on the absorption of colourless licpiids by Dr. W. 
Russell, F.R.S., and Mr. Lepraik appeared in ‘ Nature’ on the 19th August, 1880, 
it might have been thought that we were merely following in the steps of those 
gentlemen, of the scope of whose work we were not aware; we have therefore stated 
when our work commenced. It will also be seen that our work has been more 
especially confined to the infra-red region, for reasons which will presently appear, 
whilst Messrs. Russell and Lepraik turned their attention to the visible portion 
of the spectrum. 
Reasons for undertaking the research . 
The investigations of Professor Tyndall on radiant energy, and its absorption by 
carious organic compounds, led us to believe that if such marked effects were observed 
by means of the thermopile, at least as much information ought to be gathered from 
the photographic method recently brought to the notice of the Royal Society.”' The 
absorption measured by the thermopile is essentially the integration of all the absorp¬ 
tions in the different regions of the spectrum examined, and by this method it is almost 
impossible to determine the position of the several components, since the face of the 
instrument has an appreciable breadth. By the photographic method not only can an 
approximate estimate of the amount of absorption exercised by the compound be 
judged, but the exact localities of such absorptions can be indisputably fixed. 
* Bakerian Lecture “ On the Photographic Method of Mapping the least Refrangible End of the 
Spectrum,” by Captain W. de W. Abney, R.E., F.R.S., Phil. Trans., 1880, 
