The researches on which this paper is founded, were commenced in February, 1880, but were not sufficiently advanced for any communication to be made regarding them during last session. As an article on the absorption of colourless liquids 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 various 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 absorptions 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.