20 



INDIA-RUBBER TUBING. PINCH-COCKS. 



kind, from which the sulphur has been again removed, is black. 

 The grey tubing which is sold differs much in quality. Only good 

 tubing should be used in physical experiments ; although more 

 expensive, it lasts much longer than that of inferior quality. It 

 should be very elastic, so as to be capable of being stretched to 

 thrice its original length without showing the smallest cracks, and 

 should pass conveniently over a glass tube of its own external 

 diameter ; whe"n cut, the Surface of the section should be perfectly 

 smooth and bright. The quality of the black tubing is not so 

 variable, but it sticks inconveniently to glass surfaces, and its use 

 is therefore restricted. A good rubber tube should fit air-tight upon 

 a glass tube of the proper size without being tied. 



The pinch-cocks used for closing india-rubber tubes are spring- 

 clamps of brass, fig. 22, A and B. The form shown at A may be 

 easily made with the help of a pair of flat and round pliers, figs, 

 23 and 24 A brass wire about 30 cra long, and 25 inm thick, is 



FIG. 22 (A I real size, B real sick}. 



FIG. 23 

 ( real size}. 



FIG. 24 



(f 



bent into a ring c, with two parallel projections a. One of these, 

 the lower in the figure, is bent at right angles, and a small ring d 

 is made at the end ; the other projecting arm, the upper one in the 

 figure, is bent round the upright portion of the former into an ear 

 fc, then also at right angles, and finally turned at its extremity into 

 the small ring e. To produce the required elasticity the ring c 

 must be flattened by the hammer. 



Brass and copper possess the property of becoming soft and 

 flexible if exposed to a red heat ; but they regain their hardness 



