VOL. XLVIII.] PHILOSOPHICAL TRANSACTIONS. 533 



kind of polisli by rubbing it, and after a few strokes, will make the tube act in 

 the same manner as when excited at first by flannel. 



The oiled silk, when covered with chalk or whiting, will make the greased 

 rough tube act again like a polished one: but if the friction be continued till the 

 rubber is become very smooth, the electric power will be changed to that of sul- 

 phur, sealing-wax, &c. 



Thus may the positive and negative powers of electricity be produced at plea- 

 sure, by altering the surfaces of the tube and rubber; according as the one or 

 the other is most affected by the friction between them ; for if the polish be taken 

 otf one half of the tube, the difterent powers may be excited with the same 

 rubber at a single stroke. And the rubber is found to move much easier over 

 the rough, than over the polished part of it. 



That polished glass electrizes positively, and rough glass rubbed with flannel 

 negatively, seems plain, from the appearance of the light between the knuckle, 

 or end of the finger, and the respective tubes; but yet may be further confirmed 

 by observing that a polished glass tube, when excited by smooth oiled silk, if 

 the hand be kept at least 3 inches from the top of the rubber, will at every 

 stroke appear to throw out a great number of diverging pencils of electric fire, 

 as in fig. 3 ; but not one was ever seen to accompany the rubbing of sulphur, 

 sealing- wax, &c. nor was Mr. C. ever able to make any sensible alteration in the 

 air of a room, merely by the friction of those bodies; whereas the glass tube, 

 when excited so as to emit pencils, will, in a few minutes, electrify the air to 

 such a degree that, after the tube is carried away, a pair of balls, about the size 

 of the smallest peas, turned out of cork, or the pith of elder, and hung to a 

 wire by linen threads of 6 inches long, will repel each other to the distance of 

 14^ inch, when held at arm's length in the middle of the room. But their repul- 

 sion will decrejise as they are moved toward the floor, wainscot, or any of the 

 furniture; and they will touch each other when brought within a small distance 

 of any conductor. Some degree of this electric power sometimes continues in 

 the air above an hour after the rubbing of the tube, when the weather is 

 very dry. 



The electricity from the clouds, in the open air, may be discovered in the 

 same manner, if the balls be held at a sufficient distance from buildings, trees, 

 &c. as he had several times experienced, by a pair which he carried in a small 

 narrow box with a sliding cover, fig. 4, so contrived as to keep their threads 

 straight, and that they may be properly suspended, when let fall out of it ; and 

 these balls will easily determine whether the electricity of the clouds or air be 

 positive, by the decrease, or negative, by the increase of their repulsion, at the 

 approach of excited amber or sealing-wax. 



To electrify the air, or moisture contained in it, negatively ; he supported by 



