62 Captain W. de W. Ahney [March 17, 



almost bound to suppose that absorption lines in this region are 

 really dne to compound bodies of some description. Knowing the 

 results that Professor Tyndall had got with the hydrocarbon and 

 other vapours and liquids in the infra-red region by theiTQopile 

 intecrration. Colonel Festing and myself determined to see if we could 

 disintesrrate Professor Tvndall's inteOTations, and locate in the in- 

 visible spectrum the absorptions which he had noted. 



"We commenced with water, and were delighted to find that water 

 gave a very definite spectrum ; and I propose to show the method 

 adopted for this research. In front of the slit of the spectroscope, 

 which has three prisms, was placed a tube of water or other liquid, in 

 ?ome cases of the length of two feet, but more generally of six inches. 

 The crater or bright luminous patch from the positive pole of the 

 electric light was projected on the slit, the rays having to traverse 

 the liquid. The image of the spectrum was then received on a 

 sensitive plate. In this manner I propose to take the spectrum of 

 a two-foot length of water. [The photograph was taken and subse- 

 quently shown on the screen.] Before proceeding further, I will 

 a^ain show you the su]:>erior sensitiveness of the orange form of 

 bromide for blue rays over the green bromide for the dark rays. The 

 same length of spark as before shall be used, and the light from it 

 projected by means of a lens through a couple of prisms, and the 

 imac^e be focussed on an orange bromide film. The spark passes, 

 and the most refrangible end of the spectrum will be found to be 

 impressed. [The photograph was developed before the audience, 

 and at the close of the lecture thrown upon the screen.' Our next 

 attempts were with alcohol and ether, and in these we got many 

 definite absorptions, some lined and some banded, the bands being 

 more or less shaded. On trying ethyl iodide, however, we came 

 upon a spectrum which was composed of fine lines and bands with 

 comparatively sharp edges, difiering in this respect from the two 

 former spectra. The difference in composition between alcohol and 

 ethyl iodide is shown in the following diagram. 



Alcohol. Ethyl iodide. 



H H 



I I 



H— C— H H— C— H 



H— C— H 



I 

 O I 



I 

 H 



The prime difference is the presence of oxygen in the one and its 

 absence in the other. We next tried methyl iodide, and got a 

 simpler form of spectrum than the ethyl iodide. It now struck us 



I 



