358 RECENT PROGRESS IN PHYSICS. 



§ 2. Schonbein's electrical paper. — By a process similar to that used 

 in the preparation of gun-cotton, Schonhein has succeeded in converting 

 paper into a perfectly transparent substance, which, by the slightest 

 friction, becomes extraordinarily electrified, (Pogg. Ann., LXVIII, 

 159,) and which he employed in the construction of an electrical 

 machine. 



Such a substance must be in the highest degree acceptable to the 

 experimental physicist, and it is so much the more to be regretted 

 that Schonhein and Bottger have published nothing further on this 

 subject, although electrical paper is now offered for sale in Berlin. In 

 most cases the electrical paper can be replaced by thin sheets of gutta 

 perch a. 



§ 3. ElectrkAty of gutta percha. — Gutta percha is such a good in- 

 sulator, and becomes so powerfully electrified by friction, that these 

 properties of a substance, already applied to so many uses, could not 

 long remain unknown. Towards the close of the winter in 1848, 

 Dr. Hasendever, of Aachen, called my attention to this peculiarity of 

 gutta percha, and I had already used it in the construction of an 

 electrophorus, when I found, in the March number of the Phil. Mag., 

 a memoir by Faraday upon this subject, a translation of which ap- 

 peared in Pogg. Ann., (LXXIV, 154.) The following is the sub- 

 stance of Faraday's remarks upon the electric and insulating properties 

 of gutta percha : 



A good piece of gutta percha insulates as perfectly as a similar piece of 

 shellac, whether the form be that of a plate, a rod, or a mere thread; 

 but, as it is tough and pliable when cold, as well as soft when warm, 

 it serves a better purpose, in many cases, than the brittle shellac. In 

 the form of strings and bands it is an excellent suspending insulator, 

 and in that of plates it is the most convenient insulating support. 



By friction gutta percha becomes powerfully negative. Some of it 

 is sold in sheets no thicker than ordinary paper ; if a strip of this be 

 drawn between the fingers, it becomes so much electrified that it 

 adheres to the hand and attracts bits of paper. 



A plate of gutta percha makes an excellent electrophorus. 



All kinds of gutta percha are not equally good insulators. If a 

 piece of the proper kind is cut, the surface has a resinous lustre and a 

 compact appearance, while a piece of the poorer kind has not the same 

 degree of lustre, is less translucent, and looks almost like a solidified 

 cloudy fluid. 



If a piece which conducts is heated in a current of hot air or over a low 

 gas flame, pulled out, folded up and then kneaded for some time with 

 the fingers, as if to squeeze out the contained moisture, it becomes as 

 good an insulator as the best kind. 



A piece which insulates, will, if soaked in water for four days, re- 

 over its insulating power by an exposure to the air for twelve hours. 

 ^ A piece which does not insulate is greatly improved after lying for 

 eight days in a drying closet; the outer layer insulates, but a freshly 

 cut surface shows that the inside still conducts. 



Gutta percha of any kind exposed to a gradually increasing tem- 



