io ELEMENTARY LESSONS ON [CHAP. I. 



assumed to be the test of the presence of electricity. 

 Otto von Guericke first observed that sparks and flashes 

 of light could be obtained from highly electrified bodies at 

 the moment when they were discharged. Such sparks are 

 usually accompanied by a snapping sound, suggesting on a 

 smale scale the thunder accompanying the lightning spark, 

 as was remarked by Newton and other early observers. 

 Pale flashes of light are also produced by the discharge 

 of electricity through tubes partially exhausted of air by 

 the air-pump. Other effects will be noticed in due course, 

 IO. Other Sources of Electrification. The stu- 

 dent must be reminded that friction is by no means the 

 only source of electricity. The other sources, per- 

 cussion, compression, heat, chemical action, physiological 

 action, contact of metals, etc., will be treated of in Lesson 

 VII. We will simply remark here that friction between 

 two different substances always produces electrical 

 separation, no matter what the substances may be. 

 Symmer observed the production of electricity when a 

 silk stocking was drawn over a woollen one, though 

 woollen rubbed upon woollen, or silk rubbed upon silk, 

 produces no electrical effect. If, however, a piece of 

 rough glass be rubbed on a piece of smooth glass, 

 electrification is observed ; and indeed the conditions of 

 the surface play a very important part in the production 

 of electricity by friction. In general, of two bodies 

 thus rubbed together, that one becomes negatively 

 electrical whose particles are the more easily removed 

 by friction. Differences of temperature also affect the 

 electrical conditions of bodies, a warm body being usually 

 negative when rubbed on a cold piece of the same sub- 

 stance. Pe'clet found the quantity of electricity produced 

 by rubbing two substances together to be independent of 

 the pressure and of the size of the surfaces in contact, 

 but depended on the materials and on the velocity with 

 which they moved over one another. Rolling friction 

 and sliding friction produced equal effects. The quantity 



