6i4 



THE POPULAR SCIENCE MONTHLY. 



when his last experiineuts were made, that he was unable to write out 

 an account of them. On his death-bed, and indeed the very day be- 

 fore his death, his description of them was taken from his lips by Dr. 

 Mortimer, secretary of the Royal Society. 



One word of definition will be useful here. Some substances, as 

 proved by Stephen Gray, possess in a very high degree the power of 

 permitting electricity to pass through them ; other substances stop 

 the passage of the electricity. Bodies of the first class are called con- 

 ductors / bodies of the second class are called insulators. 



We cannot do better than repeat here the experiments of Gray. 

 Push a cork into the open end of your glass tube ; rub the tube, car- 

 rying the friction up to the end holding the covk. The cork will at- 

 tract the balanced latb, shown in Fig. 4, with which you have already 

 worked so much. 



But the excited glass is here so near the end of the cork that you 

 may not feel certain that the observed attraction is that of the cork. 

 You can, however, prove that the cork attracts by its action upon 

 light bodies which cling to it. Stick a pen-holder into tlie cork, and 

 rub the glass tube as before. The free end of the holder will attract 

 the lath. Stick a deal rod three or four feet long into the cork, even 

 its free end will attract the lath when the glass tube is excited. In 

 this way, you prove to demonstration that the electric power is con- 

 veyed along the rod. 



Fig. 5. 



Sec. 1. Further Inquiries on Conduction and Insulation. A lit- 

 tle addition to our apparatus will now be desirable. You can buy a 

 book of " Dutch metal" for fourpence, and a globular flask like that 

 shown in Fig. 5 for sixpence, or at the most a shilling. Find a cork, 

 C, which fits the flask ; pass a wire, W, through the cork, and bend 

 it near one end at a right angle. Stick by sealing-wax uuon the other 

 end of the wire a little plate of tin or sheet-zinc, T, about two inches 



