LESSORS IN ELECTRICITY. 



34 1 



31. A B is the glass tube, clasped by the rubber, P. P P' are 

 strips of metal furnished with rows of points. From P P wires pro- 

 ceed to the knob C, which is insulated by the horizontal stem, G, 

 from the stand of the machine. This insulating stem may be abol- 

 ished with advantage, the wires from Pand P being rendered strong 

 enough to support the ball C. At C sparks may be taken, a Leyden- 

 jar charged, the electric mill turned, while wires carried from it may 

 be employed in experiments on ignition. 



Fig. 31. 



" Seldom," says Riess, " has an experiment done so much to de- 

 velop the science to which it belongs as this of the ignition of bodies 

 by the electric spark." It aroused universal interest : the experiment 

 was repeated in all royal houses. Money was ready for the further 

 prosecution of electrical research. The experiment afterward spread 

 among the people. Klingenstierna astonished King Frederick of Swe- 

 den by igniting a spoon of alcohol with a piece of ice. Riess con- 

 siders it probable that the general interest thus excited led to the 

 discovery of the Leyden-jar, which was made soon afterward. 



Cadogan Morgan, in 1785, sought to produce the electric spark in 

 the interior of solid bodies. He inserted two wires into wood, and 

 caused the spark to pass between them : the wood was illuminated 

 with blood-red light, or wkh yellow light, according as the depth at 

 which the spark was produced was greater or less. The spark of the 

 Leyden-jar produced within an ivory ball, an orange, an apple, or 

 under the thumb, illuminates these bodies throughout. A lemon is 

 especially suited to this experiment, flashing forth at every spark as 

 a spheroid of brilliant golden light. The manner in which the lemon 

 is mounted is shown in Fig. 32. The spark occurs at s. A row of 

 eggs is also brilliantly illuminated throughout at the passage of every 

 spark from a Leyden-jar. 



