PHYSICS, EIGHTEENTH CENT.-ELECTRIC1TY. 317 



(1695 1736). Gray used as the source of electricity in his experi- 

 ments a glass tube about 3^ feet long and i inches diameter; and, in 

 order to prevent dust settling in the interior of the tube, he closed its 

 open ends with corks. The tube was simply held in the hand and 

 rubbed with a piece of cloth. One of Gray's first experiments with 

 this tube was to ascertain if the closing of the tubes with corks influ- 

 enced the development of electricity. He was unable to find any dif- 

 ference, but in the course of his experiments he observed a circum- 

 stance which put him on the track of further investigation : he noticed 

 that a feather was attracted and repelled by one of the corks exactly 

 as it would have been by the excited glass tube itself. He surmised 

 that the electric virtue had passed by contact from the tube into the 

 cork, and generalizing upon this, he conceived that perhaps electricity 

 might have the power of passing through bodies in contact, and in 

 order to test this he tried the following experiments in succession. 

 He fixed an ivory ball on the end of a piece of wood 4 inches long, 

 and thrust the other end into the cork. On rubbing the tube, he found 

 that the ivory ball attracted and repelled a feather with more vigour 

 than the cork. He then fixed the ball on longer wooden rods, to see 

 whether the effect was immediately conveyed to greater distances ; first 

 used a rod 8 inches, then one 24 inches long, and found the same effect 

 as before. He then varied the substance by taking a piece of iron 

 wire, one end inserted into the cork and the other bearing the ivory 

 ball. He found that the ivory ball attracted the feather as before, and 

 he noticed also that the ball had a stronger effect on the feather than 

 any part of the wire connecting the ball and the excited tube. Brass 

 wire acted in the same way as iron. When the piece of wire was 2 or 

 3 feet long, the vibrations produced in it by rubbing the tube making 

 the experiment more difficult to manage, Gray bethought himself of 

 hanging the ivory ball by a piece of packthread attached to the tube. 

 He found that the ball attracted and repelled light substances as 

 before. For his ivory ball he now substituted other things in succes- 

 sion : a ball of cork, one of lead ; a coin ; fire-shovel ; poker ; copper 

 tea-kettle, first empty, then filled with cold water, then with hot water ; 

 etc., and in each case the experiment succeeded as before. His ex- 

 periments, again, had for their object to determine how far the electric 

 virtue might be carried. Having by him a hollow walking-cane, 3 1 

 inches long, he fitted this into the end of his glass tube instead of the 

 cork, and found that this also conveyed the electricity to the ivory ball. 

 A solid cane had just the same effect. He then inserted part of a 

 fishing-rod, in all 14 feet long, and afterwards greater lengths of taper- 

 ing rods of cane and reed, until he at last had a ball of cork 32 feet 

 from the end of this glass, and the same result as before with regard to 

 the attractions. But the bending of this great length of rod, and the 

 vibrations which were occasioned by rubbing the tube, made this form 

 of experiment an extremely inconvenient one, and Gray therefore again 



