74 
PEOFESSOE TYNDALL ON THE ABSOEPTION AND 
at a tension of 30 inches gave these results : — 
Carbonic oxide 52 
Carbonic acid 51 "5 
And again, on the 4th of November I obtained the following relative effects : — 
Tensions. 
^ ^ ^ 
1'2 inches. 24 inches. 
O O 
Carbonic oxide 12 57 
Carbonic acid 37 54 
The same remarks apply to vapours. Methylic alcohol, for example, starts at a lower 
level than the iodide of ethyl, but ascends more quickly, and finally reaches a much 
higher elevation. The same observation may be made of chloroform in comparison with 
benzol and the iodide of ethyl. 
§ 6 . 
I haA'e now to refer to a class of facts which surprised and perplexed me when I first 
observed them. As an illustration, I will first take the case of alcohol vapour. A 
quantity of this substance, sufficient to depress the mercury gauge 0*5 of an inch, pro- 
duced 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 Avent 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 experi- 
ment 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 deflection 
of 30°; on allowing dry air to fill the tube the needle descended 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 absorp- 
tion. 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 fi’om 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 pro- 
duce 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 
