LESSONS IN ELECTRICITY. 31 



You will find it electric ; and with it you can charge your electro- 

 scope, or attract from a distance your balanced lath. 



The human body was ranked among the non-electrics. Make plain 

 to yourself the reason. Stand upon the floor and permit a friend to 

 strike you briskly with the fox's brush. Present your knuckle to the 

 balanced lath, you will find no attraction. Here, however, you stand 

 upon the earth, so that even if electricity had been developed, there 

 is nothing to hinder it from passing away. 



But, place upon the ground four warm glass tumblers, and upon 

 the tumblers a board. Stand upon the board, and present your 

 knuckle to the lath. A single stroke of the fox's fur, if skillfully 

 given, will produce attraction. If you stand upon a cake of resin, 

 of ebonite, or upon a sheet of good India-rubber, the effect will be 

 the same. 



Throw a mackintosh over your shoulders, and let a friend strike it 

 with the fox's brush, the attractive force is greatly augmented. 



After brisk striking, present your knuckle to the knuckle of your 

 friend. A spark will pass between you. 



This experiment with the mackintosh further illustrates what you 

 have already frequently observed, namely, that it is not friction alone, 

 but the friction of special substances against each other, that produces 

 electricity. 



Thus we prove that non-electrics, like electrics, can be excited, the 

 condition of success being, that an insulator shall be interposed be- 

 tween the non-electric and the earth. It is obvious that the old divis- 

 ion into electrics and non-electrics really meant a division into insu- 

 lators and conductors. 



Sec. 9. Discovery of Two Electricities. We have hitherto dealt 

 almost exclusively with electric attractions, but, in an experiment al- 

 ready referred to, Otto von Guericke observed the repulsion of a 

 feather by his sulphur globe. I also anticipated matters in the use of 

 our Dutch gold electroscope, where the repulsion of the leaves in- 

 formed us of the arrival of the electricity. 



Du Fay, who was the real discoverer here, found a gold-leaf float- 

 ing in the air to be at first attracted and then repelled by the same 

 excited body. He proved that when it was repelled by rubbed glass, 

 it was attracted by rubbed resin and that when it was repelled by 

 rubbed resin, it was attracted by rubbed glass. Hence the important 

 announcement, by Du Fay, that there are two kinds of electricity. 



The electricity excited on the glass was for a time called vitreous 

 electricity while that excited on the sealing-wax was called resinous 

 electricity. These terms are, however, improper ; because, by chang- 

 ing the* rubber, we can obtain the electricity of sealing-wax upon glass, 

 and the electricity of glass upon sealing-wax. 



Roughen, for example, the surface of your glass tube, and rub it 

 with flannel, the electricity of sealing-wax will be found upon the vit- 



