62 ELEMENTARY LESSONS ON [CHAP. i. 



LESSON VII. Other Sources of Electricity. 



57. It was remarked at the close of Lesson I. 

 (p. 10), that friction was by no means the only source 

 of electricity. Some of the other sources will now be 

 named. 



58. Percussion. A violent blow struck by one 

 substance upon another produces opposite electrical 

 states on the two surfaces. It is possible indeed to 

 draw up a list resembling that of Art. 5, in such an 

 order that each substance will take a + charge on being 

 struck with one lower on the list. Erman, who drew up 

 such a list for a number of metals, remarked that the 

 order was the same as that of the thermo-electric series 

 given in Article 381. 



59. Vibration. Volpicelli showed that vibrations 

 set up within a rod of metal coated with sulphur or 

 other insulating substance, produced a separation of 

 electricities at the surface, separating the metal from the 

 non-conductor. 



60. Disruption and Cleavage. If a card be torn 

 asunder in the dark, sparks are seen, and the separated 

 portions, when tested with an electroscope, will be found 

 to be electrical. The linen faced with paper used in 

 making strong envelopes and for paper collars, shows 

 this very well. Lumps of sugar, crunched in the dark 

 between the teeth, exhibit pale flashes of light. The 

 sudden cleavage of a sheet of mica also produces sparks, 

 and both laminae are found to be electrified. 



61. Crystallisation and Solidification. Many 

 substances, after passing from the liquid to the solid state, 

 exhibit electrical conditions. Sulphur fused in a glass 

 dish and allowed to cool is violently electrified, as may 

 be seen by lifting out the crystalline mass with a glass rod. 

 Chocolate also becomes electrical during solidification. 

 When arsenic acid crystallises out from its solution in 



