480 ELECTRICITY. 



to be formed, and kept together, it would be very singu- 

 lar that they should take fire only in the midst of a 

 drenching shower. 



2. By friction. It was by friction that electricity was 

 originally discovered, in the experiment with the amber 

 already described. It has since been ascertained that 

 many other substances, when first rubbed, will exhibit 

 the same effects. For example, if a tube of glass be 

 rubbed with a silk handkerchief, the electricities will be 

 separated; the resinous will accumulate in the hand- 

 kerchief, and the vitreous in the glass. The cause of 

 this change is not known, the fact only has been observ- 

 ed. If after the friction of the glass with the silk, the 

 two are separately examined, both will be found elec- 

 trified, but they will be in opposite states. If, however, 

 we take a rod of sealing wax, instead of glass, and rub 

 it with flannel, the effect will be reversed. That is, the 

 resinous electricity will accumulate in the sealing wax, 

 and the vitreous in the flannel. Glass by friction with 

 almost all substances becomes vitreously, or positively 

 electrified. Resins by friction with almost all substan- 

 ces become rcsinoitsly or negatively electrified. Those 

 substances called conductors of electricity will not be- 

 come electrified by friction at all. Nearly if not quite 

 all other bodies may. The reason why conductors can- 

 not be electrified by friction seems to be that, on account 

 of the ease with which the fluid passes over them, no 

 separation of the two electricities can be effected. Those 

 substances which can be electrified by friction are call- 

 ed electrics. The following is a list of the most impor- 

 tant of them : Glass, Precious Stones, Amber, Sul- 

 phur, Shell-lac, Resinous and Bituminous substance?, 

 Silk, Wax, Cotton, and dry animal substances, as Feath- 

 ers, Wool, Hair, &-c, Paper, Dry Sugar, Air and Gases. 



This method of accumulating electricity, viz. by fric- 

 tion, is the most commonly resorted to, for experiments. 

 The apparatus is called the electrical machine. It con- 

 sists of glass, either in the form of a cylinder or plate, 

 mounted in such a manner as to be turned rapidly, by a 

 crank. A rubber, made of silk is pressed against the 

 glass while in motion, and thus the vitreous electricity is 



