233 



LESSONS IN ELECTRICITY. 



special substance to make the excitement 

 strong. This \vo learn by experience. 

 It is also experience that has taught 

 us that resinous bodies are best ex- 

 cited by tianiic-], and vitreous bodies by 

 Bilk; 



Take nothing- for granted in this in- 

 quiry, and neglect no effort to render 

 your knowledge complete and sure. Try 

 various rubbers, r.nd satisfy yourself that 

 differences like that first observed by 

 Newton exist between them. 



Vary also the body rubbed. Excite 

 by friction paraftine and composite can- 

 dies, resin, sulphur, beeswax, ebonite, 

 and shellac. Also rock-crystal and other 

 vitreous substances, and attract with all 

 of them the balanced lath. A film of 

 collodion, a sheet of vulcanized india- 

 rubber, or brown paper heated before 

 the (ire, rubbed briskly with the dry 

 hand, attracts and is attracted by the 

 lath. 



Lay bare also the true influence of 

 heat in the case of our rubbed paper. 

 Spread a cold sheet of foolscap on a cold 

 board on a table, for example. If the 

 air be not very dry, rubbing, even with 

 the india-rubber, will not make them 

 cling together. Bat is it because they 

 were hot that they attracted each other 

 in the first instance ? No, for you may 

 heat your board by plunging it into boil- 

 ing water, and your paper by holding it 

 in a cloud of steam. Thus heated they 

 cannot be made to cling together. The 

 heat really acts by expelling the moisture. 

 Cold weather, if it be only dry, is highly 

 favorable to electric excitation. During 

 frost the whisking of the hand over silk 

 or flannel, or over a cat's back, renders 

 it electrical. 



The experiment of the Florentine 

 academicians, whereby they proved the 

 electric attraction of a liquid, is pretty, 

 and worthy of repetition. Fill a very 

 small watch-glass with oil, until the 

 liquid forms a round curved surface, ris- 

 ing a little over the rim of the glass. A 

 strongly excited glass tube, held over the 

 oil, raises not one eminence only, but 

 several, each of which finally discharges 

 a shower of drops against the attracting 

 glass. The effect is shown in fig. 5, 

 where G is the watch-glass on the stand 

 T, and R the excited glass tube.* 



Cause the excited glass tube to pass 



FIG. 5. 



close by your face, without touching it. 

 You feel, like Hauksbee, as if a cobweb 

 were drawn over the face. You also 

 sometimes smell a peculiar odor, due to 

 a substance developed by the electricity, 

 and called ozone. 



Long ere this, while rubbing your 

 tubes, you will have heard the " hiss- 

 ing'' and *' crackling" so often referred 

 to by the earlier electricians ; and if you 

 have rubbed your glass tube briskly in 

 the dark, you will have seen what they 

 called the "electric fire." Using, in- 

 stead of a tube, a tall glass jar, rendered 

 hot, a good warm rubber, and vigorous 

 friction, the streams of electric fire are 

 very surprising in the dark. 



6. Discovery of Conduction and Insu- 

 lation. 



Here I must again refer to that most 

 meritorious philosopher, Stephen Gray. 

 In 1729 he experimented with a gfoss 

 tube stopped by a cork. When the tubo 

 was rubbed, the cork attracted light bod- 

 ies. Gray states that he was " much 

 surprised "at this, and he " concluded 

 that there was certainly an attractive vir- 

 tue communicated to the cork." This 

 was the starting point of our knowledge 

 of electric Conduction. 



A fir stick 4 inches long, stuck into 

 the cork, was also found by Gray to at- 

 tract light bodies. He made his sticks 



* As a practical measure the watch-glass 

 ought to rest upon a small stand, and not 

 upon a surface of large area. The experi- 

 ment is particularly well suited for projection 

 on a screen. 



