356 ELECTRICITY. 



and tie a silk string C C 1) to it, which serves to 

 hold it by when it is to be set near the fire: fill 

 the middle part of this tube with resin, sealing- 

 wax, &c, then introduce two wires A A, through 

 its ends, so that they may touch the resin, or pene- 

 trate a little way into it. This done, let a person 

 hold the tube over a clear fire, so as to melt the 

 resin within it: at the same time, by connecting 

 one of* the wires A A with the outside of a charged 

 jar, and touching the other with the knob of the 

 jar, endeavour to make the discharge through the 

 resin, and you will observe, that while the resin 

 is cold, no shocks can be transmitted through it, 

 but it becomes a conductor as it melts: and when 

 totally melted, the shocks will pass through it 

 very freely. 



The Spiral Tube. 



Fig. 10. is an instrument composed of a glass 

 tube, closed with two knobbed brass caps. This 

 tube has a spiral row of small round pieces of tin- 

 foil, stuck upon its outside surface, and lying at 

 about one-thirteenth of an inch from each other. 

 If this instrument be held by one of its extremities, 

 and its other extremity be presented to the prime 

 conductor, every spark that it receives from the 

 prime conductor will cause small sparks to appear 

 between all the round pieces of tin-foil stuck upon 

 the tube, which, in the dark, affords a pleasing 

 spectacle; the instrument appearing encompassed 

 by a spiral line of fire. 



The small round pieces of tin-foil are sometimes 

 stuck upon a flat piece of glass (Fig. 11.), so as to 

 represent curve lines, flowers, letters, &c. and they 

 are illuminated after the same manner as the spiral 



