26G Chemical Instruments and Operations, 



gas to be in equilibrio with that of the external air, and con- 

 sequently to measure it with accuracy. Excepting the gage, 

 the mechanism by which the measurement is effected, is the 

 same as that of the sliding rod eudiometers for water. How- 

 ever, in addition to the stuffing box at A, there is, in the 

 mercurial eudiometers, a collar of cotton wick soaked in oil, 

 and packed by a screw, B, which includes the cotton and 

 compresses it about the rod. The object of this addition is 

 to supply oil to the rod where it enters the collar of leather ; 

 otherwise, it would soon become so dry as to allow air or 

 mercury to pass. 



Let us suppose that this eudiometer has been thoroughly 

 filled with mercury, and that it is firmly fixed in the position 

 in which it is represented in the drawing, so that the lower 

 part may descend about an inch below the surface of some 

 mercury contained in an iron cup. At C is a cock, the key 

 of which, in addition to the perforation usual in cocks, has 

 another at right angles to, and terminating in the ordinary 

 perforation. When the lever, D, attached to the key, is sit- 

 uated as it appears in the drawing, the tube containing the 

 sHding rod communicates with the receiver, but not with the 

 mercury in the cup. Supposing tlie lever moved through a 

 quarter of a circle to the other side of the glass, the tube in 

 which the rod slides will communicate at the same time with 

 the receiver, E, and the mercury. F is a steel spring, which 

 has a disk of oiled leather let mto it, so as to correspond with 

 the surface of the apex of the receiver, E, which is ground 

 as true as possible. Hence, a slight pressure from the screw, 

 G, renders the joint made between the apex of the receiver 

 and the spring, air-tight ; while at the same time the bore of 

 the cock, H, communicates with the cavity of the receiver 

 by means of a perforation through the leather and spring. 

 On the other hand, the relaxation of the screw permitting the 

 spring to rise, opens a communication between the cavity of 

 the receiver, and the external air. The cock, H, supported 

 by the spring, carries a gallows with a screw, I, which serves 

 to fasten a small brass casting, so perforated and fitted as to 

 produce a communication between the cock, H, and two 

 others, K L, with which the ends of the casting are severally 

 furnished. The cock, K, serves to open or close the com- 

 munication with the gage, M, and bell glass within the jar, 

 N. The bell glass is furnished with a cock, upon which the 

 socket, O, of the gage, screws. 



