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On a New Mercurial Gasometer and Air-pump. [June 16, 



machine the old air-pump theorem ought to hold, and by it, with the frac- 

 tion ^, I find that the fifth should give 0*007, and the sixth 0*0014 ; so 

 that the presence of adhering air is still sensible, though very shght. So 

 high a power, however, is not long maintained ; for by use, and especially 

 with oxygen, which (probably from the presence of ozone) has a peculiar 

 tendency to dirty mercury, the bell becomes soiled ; but it continues to 

 give a vacuum of 0-02, which is quite sufficient for ordinary objects. At 

 common pressure and temperature, the electric discharge through the re- 

 ceiver shows no evidence of the presence of mercurial vapour ; but at 0'02 

 it is otherwise ; the discharge is greenish white, and the spectrum shows 

 little except the lines of mercury. If the gauge were detached, perhaps this 

 vapour might be absorbed by gold-leaf. 



The apparatus acts well as a mercurial gas-holder, and can deliver 18*5 

 inches. Like all other contrivances for confining gaseous matter by mer- 

 cury, it is Hable to have its contents contaminated with air by diffusion 

 between the metal and the vessel which contains it ; but I expected that in 

 this arrangement the defect would be little felt. In order that it may take 

 place, the air must pass a distance of 17' 2 inches, of which 14" 6 is a tube 

 only 0*125 in diameter, and the rest is in a vertical direction against the 

 pressure of 2*6 inches of mercury. A single experiment will show how far 

 this avails. The bell was filled with dry hydrogen, which was found to 

 contain 0*901 of the pure gas ; it was left for ten days exposed to consider- 

 able changes of temperature, and was then found to have 0*854 ; it was 

 therefore contaminated at the rate of 005 per day. I am not aware of 

 similar measures with ordinary mercurial apparatus ; nor is this amount of 

 error very important ; but it may I believe be corrected by a means long 

 since announced by the late Professor Daniell which has been strangely 

 neglected. He proposed it to prevent the infiltration of air into barometers. 

 If the liquid metal adhered to the surface which it touches, as water would, 

 this action could not occur ; now it ivets, if I may use the word, several 

 metals, as copper or silver, but it also dissolves them, and becomes less fluixl. 

 Daniell, however, found that it does wet platinum without acting on it in 

 any injurious degree ; and advised that a ring of platinum wire should be 

 fused round the tube where it dips into its cistern. On inquiring of his 

 friend and fellow-labourer. Dr. W. A. Miller, I learn that it was completely 

 successful, but was not taken up by the opticians, and passed out of 

 memory. It is obvious that if a bit of platinum tube were cemented in 

 the vertical passage below D, it would effectually bar the diffusion. I do 

 not like to undo the joint there, which is now perfectly tight ; but I will 

 certainly, when the opportunity offers, try the experiment. 



