Miscellanies. 405 



abraded rubber itself, adheres to the firm part of the latter, chiefly 

 at the last stage of the rubbing, by an electrical attraction, and ac- 

 cordingly, when the hand is passed lightly over the rubber, the adher- 

 ing matters drop off, because the hand conducts away the electricity. 



When a piece of India rubber is pressed closely upon the brass 

 cap of a Bennet's gold leaf electrometer, and suddenly withdrawn, 

 the leaves will diverge and strike the sides of the glass ; if the rub- 

 ber is simply stretched and applied, the excitement is feeble, espe- 

 cially if slowly withdrawn, "while a smart separation causes the 

 leaves to diverge at once to their greatest extent." The production 

 of heat, when a thong of caoutchouc is held againsf the lips and sud- 

 denly pulled to the utmost, and at the same moment pressed hard, is 

 well known. 



According to Dr. J. K. Mitchell, the caoutchouc, even in the ex- 

 treme thinness to which he reduces it by his peculiar mode of ex- 

 panding it, by blowing after immersion in ether, entirely prevents the 

 passage of the electric spark from the prime conductor; it is how- 

 ever probable that it would be lacerated by the discharge from a 

 powerful battery. 



The remainder of Mr. Johnson's paper, we quote entire. 



" The fact, however, that it has a power of resisting to a consider- 

 able extent, points it out as a good medium to be interposed between 

 the two surfaces of the condenser, or substituted in some form for 

 the Leyden phial. 



" For this purpose, a piece of gum, reduced to a very thin sheet, 

 may be interposed between two sheets of tin foil and laid upon a ta- 

 ble ; a thicker sheet of gum may then be laid upon the upper sheet 

 of foil, so that the edge of the latter should be at some distance from 

 that of the former. The whole may then be rolled up into a coil, 

 allowing a small part of the included tin foil to project out at one 

 end of the roll. A charge may now be given to this apparatus, and 

 a shock obtained by connecting the outer sheet of tin with the part 

 of the inner, projecting at the end. 



"A disk of metal may be covered with a thin sheet of caoutchouc 

 and another disk furnished with an insulating handle placed above it; 

 this apparatus will serve all the purpose of the ordinary condenser. 



" I have stretched a piece of gum upon a circular piece of board, 

 six inches in diameter, with a coat of tin foil underneath ; on rubbing 

 this with flannel, it becomes highly electrified, and if a plate, like the 



Vol. XX.»=No. 2. 52 



