LESSONS IN ELECTRICITY. 613 



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, rising 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. 



Cause the excited glass tube to pass close by your face, with- 

 out 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 " hissing " 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, 

 instead of a tube, a tall glass jar, rendered hot, a good warm rub- 

 ber, and vigorous friction, the streams of electric fire are very sur- 

 prising in the dark. 



Sec. 6. Discovery of Conduction and Insulation. Here I must 

 again refer to that most meritorious philosopher, Stephen Gray. In 

 1729, he experimented with a glass tube stopped by a cork. When 

 the tube was rubbed, the cork attracted light bodies. Gray states 

 that he was " much surprised " at this, and he " concluded that there 

 was certainly an attractive virtue communicated to the cork." This 

 was the starting-point of our knowledge of electric conduction. 



A fir-stick four inches long, stuck into the cork, was also found 

 by Gray to attract light bodies. He made his sticks longer, but 

 still found a power of attraction at their ends. He then passed on 

 to packthread and wire. Hanging a thread from the top window 

 of a house, so that the lower end nearly touched the ground, and 

 twisting the upper end of the thread round his glass tube, on briskly 

 rubbing the tube, light bodies were attracted by the lower end of 

 the thread. 



But Gray's most remarkable experiment was this : He suspended 

 a long hempen line horizontally by loops of packthread, but failed to 

 transmit through it the electric power. He then suspended it by 

 loops of silk and succeeded in sending the " attractive virtue " 

 through 765 feet of thread. He at first thought the silk was efiectual 

 because it was thin ; but, on replacing a broken loop by a still thin- 

 ner wire, he obtained no action. Finally, he came to the conclusion 

 that his loops were effectual, not because they wei'e thin, but because 

 they wei'e silJc. This was the starting-point of our knowledge of 

 insulation. 



It is interesting to notice the devotion of some men of science to 

 their work. Dr. Wells finished his beautiful essay on "Dew " when he 

 was on the brink of the grave. Stephen Gray was so near dying, 



