Dr. Robinson on a New Mercurial Gasometer and Air-pump. 239 



receiver must be in its place, or the opening of R must be closed by 

 a piece of flat glass ; the bell must be filled by the plunger, and 

 made, by (r) and by opening Tc, to communicate with the jar N. 

 The mercury will rise in that to its neck, and sink in A ; fill A again, 

 pass gas into N, and, by carefully working the key, draw it into A 

 till that is full. As this gas will be mixed with the air of the vessels 

 and passages, it must be expelled, and A refilled till its purity is cer- 

 tain. If it be noxious, it must be conducted into some absorbent 

 fluid by an elastic tube, slipped on the a end of the cock ; which will 

 also convey the gas to any vessel. 



If it be required to fill a receiver for experiments in an atmo- 

 sphere of gas either at common pressure or a less one, it may either 

 be exhausted by an air-pump connected with K, and filled from A, 

 or exhausted by A and filled from N. The former can only be done 

 with gases which have no action on brass. 



These operations seem complicated when described with so much 

 detail, but in practice they are very easy, and their result is good. 

 Some precautions, however, are required to ensure it. The bottom 

 of the bell-cock and of its key must be ground, so as to leave no 

 shoulder or hollow in which air may be entangled when the bell is 

 filled. Every part of the metal work must be air-tight ; this can 

 only be secured by covering, not only its joints, but its whole surface 

 with several coats of varnish-paint — best of white lead. When the 

 first coat is applied, on exhausting the apparatus, every hole or pore 

 is revealed by an opening in the paint (often almost microscopic), 

 which must be filled up as it forms till all is tight. It is almost 

 needless to mention that the whole must be perfectly dry. If the 

 bell be filled a few times with undried air, enough of moisture will 

 adhere to its walls to prevent an exhaustion of more than 0*1 inch. 

 In such a case it must be dried by drawing air into it through sul- 

 phuric acid, and this repeatedly. Moisture also occasionally finds 

 its. way into a part still more troublesome, into the passage which 

 connects the bell and cylinder ; it is probably condensed there when 

 the mercury is colder than the atmosphere." I remove this by connect- 

 ing the tube of K with a desiccator ; setting C to (r), opening K and 

 E, and by working the air-pump drawing a stream of dry air into D, 

 which bubbles up through the mercury in the passage, and at last 

 sweeps away all trace of water and its vapour. In this operation it is 

 necessary to remove a portion of the mercury, as otherwise it would 

 be sacked into the pump ; indeed this mischief might occur in ordi- 

 nary work by some mistake in the manipulation — for instance, by 

 leaving E open with (a). To prevent the possibility of this, D is 

 connected with the pump by a mercury trap, easily imagined, which 

 intercepts any of that metal that might come over. And lastly, the 

 interior of the bell must be perfectly clean if the highest degree of 

 exhaustion is required. This state is obtained by washing it with 

 strong nitric acid, then with distilled water, and when quite dry wiping 

 it with linen, from which all traces of soap or starch have been re- 

 moved by boiling it in rain-water. Thus we reduce to a minimum 

 the film of air which adheres to the bell even when filled with mer- 



