322 Dr. Robinson on a New Mercurial Gasometer [June 16, 



vacuum; this vessel may be made to communicate with a receiver, and 

 abstract from it a portion 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 filled 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 position, and thus allow the metal to fall from the other, or 

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

 but it must be rather unwieldy ; and my 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 re- 

 moving in any way that pressure ; and an elf active means of doing this is 

 supplied by the common air-pump ; more tedious certainly than the mecha- 

 nical means above mentioned, but far more manageable ; and as any mer- 

 curial pump must be slow in its working, while it is only required for spe- 

 cial purposes, this defect is not of much importance, and moreover is com- 

 pensated 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 mahogany, 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 flanch, 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 figure), the joint is air-tight. 

 The bell A is 2 inches in diameter and 6*5 high ; it has a tubulure at the top, 

 in which is ground a glass cock C, whose construction is shown in fig. 2. 

 The key of it is pierced from its bottom to a level with the bore, with 

 which this perforation communicates occasionally by a lateral opening. In 

 the position of the figure, it will be seen that the bell communicates with 

 the branch a ; if the key be turned half round, it is connected with the 



