LESSONS IF ELECTRICITY. 



unite with explosion to form water, when 

 an electric spark is pnssed through them. 



Mr. Cottrell has also mounted his glass 

 -tube so as to render friction in both direc- 

 jtions available. The tube-machine is 

 represented in fig. 46. A D is the glass 

 tube, ciasped by the rubber, n. p p 1 arc 

 two strips of metal furnished with rows 

 of points. From p p' wires proceed to 

 the knob c, which is insulated by the 

 horizontal stem, o. This insulating stem 

 may be abolished with advantage, the 

 wires from p and p' being rendered strong 

 enough to support the ball c. At c 

 sparks may be taken, a Ley den jar 

 charged, the electric mill turned, while 

 wires carried from it may be employed 

 in experiments on ignition. I however 

 strongly recommend to your attention 

 the more simple rubber shown in fig. 44. 



" Seldom," says Hies*, " has an ex- 

 periment done so innch to 'develop the 

 science to which it belongs a* this of the 

 ignition of bodies by the electric sparks." 

 It aroused universal interest ; and was 

 repeated in all Royal houses. Money 

 was ready for the further prosecution of 

 electrical research. The experiment 

 afterward spread among the people. 

 Jliesw considers it probable tlu-it the gen- 

 eral interest thus excited led to th dis- 

 covery of the Ley de u jar, which was mado 

 soon after vr&rd. 



Fio. 47. 



Klingenstierna astonished King Fred- 

 erick of Sweden by igniting a spoon of 

 alcohol with a piece of ice. With Cot- 

 trell's rubber and bisulphide of carbon 

 this striking experiment is easily made, 

 and you ought to render your knowl- 

 edge complete by repeating it. At 

 every stroke of the rubber the spark 

 from the end of a pointed rod of ice in- 

 fallibly sets the bisulphide) on tire. 



Cadogan Morgan, in 1785, sought to pro- 

 duce the electric spark in the interior of 

 solid bodies. He inserted two wires into 

 wood, and caused the spark to pass be- 

 tween them: the wood \rns illnminated with 

 blood-red light, or with yellow light, ac- 

 cording as the depth at which the spark 

 was produced wa~s greater or less. Tlia 

 spark of the Leyden jar produced within an 

 ivory ball, an orange, an apple, or under tin; 

 thumb, illuminates these bodies through- 

 out. A lemon is especially suited to this 



ffra, 48. 



