1863.] 679 



It would, I think, be interesting to determine, on the more trans- 

 parent portions of glacier ice, by the simple and easily applied test of 

 polarized light, whether a definite crystalline structure prevail in its 

 interior, and if so, in what direction the axis lies in relation to the 

 lines of fissure in the crevasses. Nor is there any reason why the 

 idea above thrown out respecting the mutual modification of structure 

 of two masses cemented by regelation, at or near their plane of junc- 

 tion, should not be subjected to a similar test. 



XX. " On the Absorption and Radiation of Heat by Gaseous 

 and Liquid Matter." Fourth Memoir. By JOHN TYN- 

 DALL, F.R.S. Received June 18, 1863. 



In his former researches on the absorption and radiation of heat 

 by gaseous matter, the author compared different gases and vapours 

 at a common thickness with each other. In the first part of the 

 present communication he determines, in the case of several gases and 

 vapours, the absorption effected by different thicknesses of the same 

 gaseous body. His least thickness was 0*01 of an inch, and his 

 greatest 49*4 inches ; thus the thickness varied from 1 to nearly 5000. 

 The apparatus employed for the smaller thicknesses consisted of a 

 hollow cylinder, with its end closed by a plate of rock-salt ; into this 

 fitted a second hollow cylinder, with its end closed by a second 

 plate of salt. One cylinder moved within the other as a piston, and 

 by this means the plates of salt could be brought into flat contact 

 with each other, or separated to any required distance. The dis- 

 tance between the plates was measured by means of a vernier. The 

 cylinder was placed horizontal, being suitably connected with the 

 front chamber used in the author's former researches, and the 

 source of heat employed was a copper plate, against which a steady 

 sheet of gas-flame was caused to play. 



The absorptions of carbonic oxide, carbonic acid, nitrous oxide, 

 and olefiant gas were determined by this apparatus, and such dif- 

 ferences as might be anticipated from former experiments were 

 found. Olefiant gas maintained its great supremacy over the others 

 at all thicknesses. A layer of this gas, O'Ol of an inch thick, effected 

 an absorption of about 1 per cent, of the total radiation. To show 



