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



two bent arms, which diverge from a centre, as a pair of 

 compasses, and when adjusted are held tight by a screw at 

 the centre. A reel of tine pendulum wire is fixed at one 

 end by a screw, and at the other by a small pair of nippers. 

 The whole is of baked wood, with glass supports. 



Price, $3.00. 



Fig. 349. Ignition of Charcoal Points in Chlorine. 



(Fig. 349.) This consists of a glass 

 globe, or other vessel, having two necks, 

 each having brass caps, the lower one a 

 stop-cock for connecting with the air 

 pump, and the upper one a brass rod 

 sliding in a stuffing box, on the interior 

 end of which is a pair of forceps for hold- 

 ing a charcoal point, another similar point 

 being fixed to the stop-cock below. To 

 use, fill the vessel with chlorine gas, and 

 adjust the wires, so that their points shall 

 nearly touch each other. When the 

 electric current is made to pass through 

 the wires, the charcoal points will be ig- 

 nited, becoming of a red heat, yet the chlorine will not be 

 affected, however long the action may be pursued. 



Price, $6.00. 



Fig. 350. The Sphere and Point. (Fig. 350.) Faraday's 

 | hollow brass sphere and tubular handle, with 

 pointed sliding wire within, for the purpose of 

 showing the influence of surfaces in electrical dis- 

 charges. When the sphere is employed a bright 

 spark is observable at each discharge, provided the 

 point of the sliding wire be within the sphere, but 

 if it projects without, the discharge changes its form 

 to that of a brush. Price, $1.50. 



Biot's Movable Hemispheres and Ball. (Fig. 

 351, next page.) For showing that electricity re- 

 sides on the surface only. When a substance be- 

 comes charged with electricity, it is extremely 

 probable that the fluid is confined to its surface, 

 or, at any rate, that it does not penetrate into the 

 mass to any extent. A ball formed of any material will be 



