236 Royal Society : — 



There have been .several attempts made to exhaust by means of 

 mercury, the chief of them with which I am acquainted being those 

 of Close (Nicholson's Journal, 4to, hi. p. 264), Edelcrantz (Nicholson, 

 8vo, vii. ]). 188), Traill and Children (Nicholson, xxi. pp. 63 & 

 161), and that of Geisler, which he uses in preparing the beautiful 

 vacuum-tubes which bear his name. In all the principle is the 

 same. A vessel is rilled with mercury, which is made to descend 

 from it, leaving in it a Torricellian vacuum ; this vessel may be 

 made to communicate with a receiver, and abstract from it a por- 

 tion of the gas which fills it ; and by repeating the process the 

 receiver can be exhausted as by successive strokes of an air-pump. 

 In the two first instruments to which I have referred, the descent of 

 the mercury is produced by lifting a plunger which fills one leg of 

 an inverted siphon, the vacuum vessel being at the top of the other 

 leg. On depressing the plunger, the mercury is again forced up to 

 fill that vessel ; and of course both legs must be longer than the 

 barometric column. In the two next, the receiver itself is rilled with 

 mercury, which, by opening a cock, falls through a tube of sufficient 

 length into a cistern below. Here the stroke (so to call it) cannot 

 be repeated. In Geisler's the bend of the siphon is of vulcanized 

 caoutchouc, so that one leg can be inclined down to a horizontal posi- 

 tion, and thus allow the metal to fall from the other, or when raised 

 to the vertical position fill it again. This I believe acts well, but it 

 must be rather unwieldy ; and ray practical acquaintance with the 

 working of tubes of that material has made me suspicious of their 

 tightness and permanence under such circumstances. 



As in all these cases the mercury is supported in the vacuum- 

 vessel by atmospheric pressure, it is obvious that its descent will be 

 produced by removing in any way that pressure ; and an effective 

 means of doing this is supplied by the common air-pump; more 

 tedious certainly than the mechanical means above mentioned, but 

 far more manageable ; and as any mercurial pump must be slow in its 

 working, while it is only required for special purposes, this defect is 

 not of much importance, and moreover is compensated by some 

 special advantages. 



But besides bringing down the mercury, means must be provided 

 for raising it again. My first plan was to do this by condensed air, 

 the same syringe which made the exhaustion having its action 

 reversed by a well-known arrangement. It worked extremely well, 

 was lighter, and required less mercury than the contrivance which I 

 finally adopted ; but it is less convenient for gasometric work, as the 

 syringe must be worked while gas is delivered. The machine in its 

 present form is shown in fig. 1 . Its base is a stout piece of maho- 

 gany, 21 inches by 10*5, with a rim round it 0*5 deep to prevent 

 the loss of any spilled mercury, and handles at the ends by which it 

 can be transported. To this is firmly fixed the iron stand B, 3*5 

 high, 4 in diameter above; its upper surface is carefully trued to a 

 rlanch, in which is cemented the vacuum- bell A, so that when the 

 touching surfaces are lightly smeared with a mixture of lard and wax 

 and screwed together by the six screws (some of which appear in the 



