VOL. LXXII.] PHILOSOPHICAL TRANSACTIONS. 2(ii 



plate, when it is to be used for our experiment, should be quite free from any the 

 least electricity, otherwise the experiments cannot be depended on. If therefore 

 the resinous plate has been excited before, so as to remain in some measure elec- 

 trified, all possible care should be taken to deprive it of that electricity, which 

 however is not easily done. The most effectual method of doing it, is to expose 

 the resinous plate to the hot rays of the sun or to a fire, so that its surface may 

 be slightly melted, by which means it will entirely lose its electricity.* The 

 flame of a candle, or of a piece of paper, will easily deprive the resin of its elec- 

 tricity, if its surface be passed over the flame. To observe whether the resinous 

 plate is quite free from any electricity, the metal plate must be laid upon it, 

 there it must be touched with a finger, and afterwards, being lifted up after the 

 usual manner, it must be presented to a fine hair ; for if the hair is not attracted, 

 it may be concluded that the resinous plate has no electricity, and consequently 

 the apparatus is fit to be used as a condenser of electricity. 



Whenever the atmospherical conductor by itself gives sufficiently strong signs 

 of electricity, then there is no occasion to use our condensing apparatus. Besides, 

 when the electricity is strong, it often happens that part of the electricity of the 

 metal plate is impressed on the resin, in which case the apparatus acts as an elec- 

 trophorus, and consequently is unfit for our purpose. To avoid such an incon- 

 venience, Mr. V.'s plan is to substitute to the resinous plate a plane, which 

 should not be a perfect electric, or quite impervious to electricity, but which 

 should be an imperfect conductor, such as might hinder, in a certain degree 

 only, the (ree passage of the electric fluid through its substance. There are 

 many conductors of this kind ; as, for instance, a clean and dry marble slab, a 

 plate of wood, likewise clean and very dry, or covered with a coat of varnish, 

 or wax; and the like. The surface of those bodies does not contract any elec- 

 tricity ; or if any electricity adhere to them, it vanishes soon, on account of their 

 semi-conducting nature ; for which reason they cannot answer the office of an 

 electrophorus, and therefore are more fit to be used as condensers of electricity. 

 On the other hand, care should be taken, in choosing the above-mentioned 

 plane, that it be not too much of a conducting nature, or capable of becoming 



* It has been believed for a long time, that to heat, and especially to melt, sulphur and resins 

 was sufficient to excite in them some electricity; but except the tourmalin and some other stones 

 which are really excited by heat alone, the resins and sulphur never become electrified by that means 

 except when they have by some means or other suffered any friction. The mistake, as Beccaria ob- 

 served, was occasioned by this, viz. that even the least friction of the hand, or other body, is suffi- 

 cient to excite such substances in those favourable circumstances, without which friction, those sub- 

 stances, melted and left to cool by themselves, are so far from acquiring any electricity, that they 

 lose every vestige of it in case they were excited before the fusion, as may be easily proved by ex- 

 periment : nor ought this to appear wonderful, since fusion or a strong degree of heat renders every 

 body a conductor of electricity. — Orig. 



VOL. XV. M M 



