Apparatus and Processes. 251 



The additional valve cock, represented in the figure, gives the op- 

 tion of introducing dry hydrogen for the purpose of washing out at- 

 mospheric air, as described in the process for silicon. 



3. Description of the Valve Cock r a perfectly air-tight substitute for 



the common Cock, alluded to in the preceding articles. 



This figure is intended to illustrate the construction of a substitute 

 for a common cock, which I have been accustomed to call a valve 

 cock* It was devised by me about twenty years ago, among a num- 

 ber of other analogous contrivances, and seems upon the whole less 

 liable to fail than any other which I have tried. The engraving 

 represents a longitudinal section of the valve-cock. At a is a piston? 

 with a collar enclosed in the stuffing box 6, so as to be rendered air- 

 tight by means of oiled leather. Hence the piston may be turned 

 or made to revolve on its axis, while incapable of other motion. 

 Upon the end of the piston a thread for a screw is cut which fits intt> 

 a female screw in the brass prism c, so as to cause this prism to 

 approach to, or retreat from a bearing, covered by leather, in the 

 centre of which there is a perforation o o communicating with one 

 of the orifices of the instrument. This orifice is surrounded by the 

 male screw d, so that by means of this screw, the valve-cock may 

 be fastened into an appropriate aperture, properly fitted to receive it r 

 subjecting an interposed leather to such pressure, as to create with 

 it an air-tight juncture. The prism c, has two of its four edges cut 

 off (see fig. 2,) so as to allow a free passage by it, reaching to the 

 lateral perforation terminating in another orifice, over which there 

 is a gallows screw, g. By means of this gallows screw, when requi- 

 site, a brass knob, such as that represented by a fig. 3, soldered to a 

 leaden pipe, may be fastened to the valve cock. The juncture is ren- 

 dered air-tight by the pressure of the screw in the gallows, upon a 

 leather which is kept in its place, by means of the nipple n. 



The method last mentioned, of producing an air-tight juncture,, 

 was contrived by me about seven years ago, and proves to be of very 

 great utility. There is no other mode with which I am acquainted, 

 of making a perfectly air-tight communication, between the cavities- 

 previously separate, at all comparable to this in facility. 



