240 Boyd Society. 



eury, and lessens its vacuum. When all these precautions were taken, 

 I found that with a receiver containing 3*7 inches, the fifth opera- 

 tion Drought the gauge (which had heen similarly cleaned and care- 

 fully boiled) down to O'Ol. The sixth brought it still lower, but my 

 present means of measurement* are not sufficient to determine the 

 precise amount. In this machine the old air-pump theorem ought 

 to hold, and by it, with the fraction '^, 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 slight. 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 - 02, which is quite sufficient for ordinary objects. At 

 common pressure and temperature, the electric discharge through 

 the receiver shows no evidence of the presence of mercurial vapour ; 

 but at 002 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 mercury, it is liable 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 mer- 

 cury. 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 considerable changes 

 of temperature, and was then found to have 0*854 ; it was there- 

 fore 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 infil- 

 tration 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 fluid. 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 effec- 

 tually bar the diffusion. I do not like to undo the joint there, 

 which is now perfectly tight ; but I will certainly, when the oppor- 

 tunity offers, try the experiment. 

 * A micrometer microscope put in the place of the telescope of my theodolite. 



