Mr. Daniell on the Barometer. 85 



dent, I laid it aside, and thought no more of it till the present 

 experiments recalled it to my recollection. By a singular coinci- 

 dence I was, about this time, informed that the barometer of the 

 Royal Society had assumed a very remarkable appearance, and 

 that the mercurial column, which was originally perfectly bright 

 and compact, now seemed dull and speckled. I immediately pro- 

 ceeded to examine it carefully, and I at once perceived that it was 

 copiously studded with minute air-bubbles. As far as the mer- 

 cury was exposed to view, the specks could be traced decreasing 

 in size, from the upper to the lower part. The manner in which 

 this instrument is fixed rendered it impossible to suspect that this 

 air could have obtained admission by any accident ; for, unlike 

 the mountain barometer, the column of metal is exposed to no 

 oscillations but such as arise from differences of atmospheric 

 pressure. I was myself quite satisfied, and those who have read 

 the account of the precautions taken in filling the tube will also, 

 I think, be satisfied, that this air was not left at its original 

 construction. I now leave it to the candid to judge, whether 

 " the cause which I have assigned for the appearance of this 

 elastic matter in barometer tubes has not already been proved 

 by the experiments which I have detailed." 



While I was occupied with these considerations, and suf- 

 ficiently vexed to find that all my care had been thrown away to 

 prevent my adopting that opinion without very strong grounds, 

 it occurred to me that I had, in the course of my experience, 

 observed a phenomenon, which was calculated to throw some 

 light upon the present question ; namely, that gases were more 

 readily preserved from mixture with atmospheric air over water 

 than over mercury. I was unable to refer to any notes of expe- 

 riments to confirm this suspicion, but I proposed the question to 

 Mr. Faraday, who, I made no doubt, from his great accuracy 

 and experience, must have made the observation, if it were 

 founded in fact. Without at the time having any knowledge of the 

 ulterior object which I had in view, heat once answered me, that 

 mercury, he believed, would not confine gases for a long pe- 

 riod so well as water ; and he thought, that by referring to his 

 note-book, he could furnish me with the particulars of a case in 



