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XLV. On the Absorption and Radiation of Heat by Gaseous 

 Matter. — Second Memoir. By John Tyndall, F.R.S., 

 Professor of Natural Philosophy in the Royal Institution. 



[Continued from p. 28/.] 



§ 6. HAVE now to refer to a class of facts which surprised 



A and perplexed me when I first observed them. As an 

 illustration, I will first take the case of alcohol vapour. A quan- 

 tity of this substance, sufficient to depress the mercury gauge 0*5 

 of an inch, produced an absorption which caused a deflection of 

 72° of the galvanometer needle. 



While the needle pointed to this high figure, and previously 

 to pumping out the vapour, I allowed dry air to stream into the 

 tube, and happened while it entered to observe the effect upon 

 the galvanometer. The needle, to my astonishment, sank 

 speedily to zero, and went to 25° at the opposite side. The 

 entry of the almost neutral air here not only abolished the 

 absorption previously observed, but left a considerable balance in 

 favour of the face of the pile turned towards the source. A 

 repetition of the experiment brought the needle down to zero, 

 and sent it to 38° on the opposite side. In like manner a very 

 small quantity of the vapour of sulphuric ether produced a deflec- 

 tion of 30° ; on allowing dry air to fill the tube the needle de- 

 scended speedily to zero, and swung to 60° at the opposite side. 



These results both perplexed and distressed me; for I imagined, 

 on first observing them, that I had been throughout dealing with 

 an effect totally different from absorption. I thought, at first, that 

 my vapours had deposited themselves in opake films on my plates 

 of rock-salt, and that the dry air on entering had cleared these 

 films away, and allowed the heat from the source free transmission. 



But a moment's reflection dissipated this supposition. The 

 clearing away of such a film could at best but restore the state 

 of things existing prior to its formation. It might be conceived 

 of as bringing the needle again to 0°; but it could not possibly 

 produce the negative deflection, which, in the case of ether 

 vapour, amounted to the vast amplitude of 60°. Nevertheless 

 I dismounted the tube, and subjected the plates of salt to a 

 searching examination. I satisfied myself thus that no such 

 deposition as that above surmised took place. The salt remained 

 perfectly transparent while in contact with the vapour. 



Some of the experiments recorded in the Bakerian Lecture for 

 this year (1860) had taught me that the dynamic heating of the 

 air when it entered the exhausted tube was sufficient to produce 

 a very sensible radiation on the part of any powerful vapour con- 

 tained within the tube, but I was slow to believe that the enor- 

 mous effect above described could be thus accounted for. My 

 first care was to determine the difference of temperature between 



