NATURAL 1'HILOSOPHY. 113 



percha very useful in electrical experiments. Its use depends upon 

 the high insulating- power which it possesses under ordinary condi- 

 tions, and the manner in \vhich it keeps this power in states of the 

 atmosphere which make the surface of glass a good conductor. All 

 gutta-percha is not, however, equally good, as it comes from the 

 manufacturer's hands ; but it does not seem difficult to bring it 

 into the best state. A good piece of gutta-percha will insulate 

 as well as an equal piece of shell-lac, whether it be in the form 

 of a sheet, or rod, or filament; but being tough and flexible when 

 cold, as well as soft when hot, it will serve better than shell-lac in 

 many cases where the brittleness of the latter is an inconvenience. 

 Thus it makes very good handles for carriers of electricity in experi- 

 ments on induction, not being liable to fracture ; in the form of a thin 

 band, or string, it makes an excellent insulating suspender ; a piece 

 of it in sheet makes a most convenient insulating basis for any thing 

 placed on it. It forms excellent insulating plugs for the stems of 

 gold-leaf electrometers when they pass through sheltering tubes, and 

 larger plugs supply good insulating feet for extemporary electrical ar- 

 rangements. Cylinders of it, half an inch or more in diameter, have 

 great stiffness, and form excellent insulating pillars. In these, and in 

 many other ways, its power as an insulator may be useful. 



Because of its o-ood insulation, it is also an excellent substance for 



^j * 



the excitement of negative electricity. It is hardly possible to take 

 one of the soles sold by the shoemaker out of paper, or into the hand, 

 without exciting it to such a degree as to open the leaves of an elec- 

 trometer, one or more inches ; or if it be unelectrified, the slightest pas- 

 sage over the hand or face, the clothes, or almost any other substance, 

 gives it an electric state. Some of the gutta-percha is sold in very 

 thin sheets, resembling in general appearance oiled silk ; and if a strip 

 of this be drawn through the fingers, it is so electric as to adhere to 

 the hand or attract pieces of paper. The appearance is such as to 

 suggest the making a thicker sheet of the substance into a plate elec- 

 trical machine, for the production of negative electricity. 



Then, as to inductive action through the substance, a sheet of it is 

 soon converted into an excellent electrophorus ; or it may be coated 

 and used in place of a Ley den jar ; or in many of the other forms of 

 apparatus dependent on inductive action. 



With respect to that gutta-percha which is not in good electrical 

 condition (and which has constituted about one-half of that which, 

 being obtained at the shops, has passed through Dr. Faraday's hands), 

 it has either discharged an electrometer, as a piece of wood or paper 

 would do, or it has made it collapse greatly by touching, yet has on 

 its removal been followed by a full opening of the leaves again. The 

 latter effect Dr. Faraday has traced and referred to a conducting por- 

 tion within the mass, covered by a thin external non-conducting coat. 

 When a piece which insulates well is cut, the surface exposed has a 

 resinous lustre, and a compact character that is very distinctive ; 

 whilst that which conducts has not the same degree of lustre, appears 

 less translucent, and has more the aspect of a turbid solution solidified. 

 Both moist steam-heat and water-baths are believed to be used in its 



