16 PNEUMATO-CHEMICAL APPARATUS- 



tained in it. If absorption takes place, either iri 

 the retort or vessels, the external air enters through, 

 these tubes, to fill the vacuum which is occasioned 

 by the absorption, and no water comes into the 

 vessels. 



Large vessels for containing air, and expelling 

 any given quantity, are called gazometers. They 

 are of various constructions ; one of the best is the 

 following : AB (fig. 2.), is a cylindrical vessel of 

 tin, japanned, nearly filled with water, and having 

 a tube, C, in the middle, open at top, and branching, 

 to communicate with the cock, D. Within this 

 vessel there is another cylindrical vessel, generally 

 of glass, of smaller size, F, open at bottom, which 

 is inverted and suspended by the lines e e, which 

 go over the pulliesj^j^j^j^ and have weights # g, 

 attached to them, to balance the vessel, F. While 

 the cock D remains shut, if the vessel F be pressed 

 downwards, the air included within it will remain 

 in the same situation, on the principle of the div- 

 ing-bell ; but if the cock be opened, and the vessel, 

 F, be pressed down, the air included within it will 

 escape through the cock, and if a blow-pipe be at- 

 tached to this cock, a stream of the gas may be 

 thrown upon lighted charcoal, or any other body. 

 By means of the graduated rod, //, also, the quan- 

 tity thrown out is exactly ascertained : this rod is 

 so divided as to express the contents of the inner 

 vessel in cubic feet, &c. This instrument also an- 

 swers for breathing any of the gases, by applying a 

 mouth-piece to the cock. To render it more 

 portable, the weights g g, are sometimes included 

 in the uprights ii, which are hollow, and wide 

 enough to receive them. Sometimes, also, there 

 is another branch from the bottom of the pipe, in 



