76 BENJ. PIKE'S, JR., DESCRIPTIVE CATALOGUE. 



gas to be collected. The gas rises up through the mercury 

 and occupies the upper arm of the tube, while the mercury 

 which is displaced drops into a dish put underneath to 

 catch it. Price, 37 to 75cts. 



Fig. 523. 



Fig. 521. 



Stop-Cock. (Fig. 524, as above.) The ordinary name 

 given to the small brass taps or cocks used in pneumatic 

 apparatus, and of which the figures are representations. 

 The first figure shows the common stop-cock, which has a 

 male screw at each end. The figure on the side of it is a 

 connecting piece, used to join one stop-cock to another, 

 when it is necessary to transfer gases through or into them. 

 The lower figure shows a stop-cock so contrived at one end, 

 that a bladder may be firmly united to it ; the other end 

 having a small screw, to which a tube or other apparatus 

 may be attached. Price, $0.75. 



" large, for air pump apparatus, $1.00. 

 " connectors, $0.25 to $0.50. 



Binding Screw. A term applied by opticians to 

 denote the screws by which the wires of galvanic 

 batteries, electro -magnetic apparatus, and other 

 similar things are bound together, during the time 

 of their action. As good metallic contact is neces- 

 sary in electro-magnetic experiments, the extreme 

 point of the upper screw, as well as the end of the 

 wire to be inserted, should be made bright, when wanted 



for use in delicate experiments. 



Price, 25cts. 



Bell-Glass and Bladder Apparatus. (Fig. 525, next page.) 

 One of the most extensively useful and convenient articles 

 of chemical apparatus is what is termed the bell-glass and 



