20ti PHILOSOPHICAL TRANSACTIONS. [ANNO 1/82. 



so in a very short time, it being quite necessary, that the electricity should find 

 a considerable degree of resistance in going through its substance. In choosing, 

 or in preparing, such a plane by drying, or otherwise, it is better to render it too 

 near to than too far from the nature of a non-conductor. A marble slab, or a 

 board properly dried, answers admirably well, and is preferable to any other 

 plane : otherwise the resinous plate of an electrophorus is preferable to a common 

 table or marble slab not prepared ; for these bodies, having in some measure 

 imbibed moisture, conduct much better than is necessary. 



For this purpose it is better to use a flat piece of marble, and to grind it 

 against the metal plate, till they coincide so well as to show a sensible cohesion 

 between them. Afterwards the piece of marble should be exposed for several 

 days to the heat of a warmed place, such as an oven, a chimney, &c. to expe] 

 the moisture, and to render it quite fit for our experiments. The marble, thus 

 prepared, will continue dry for a considerable time, unless it be long exposed to 

 very damp air. As for the small quantity of moisture which the marble may 

 accidentally and superficially attract, it may be removed by exposing it to the 

 sun, or to a fire, or even by wiping it with a dry and clean cloth, previous to 

 the performing of experiments. It is always advantageous to warm the marble 

 previous to the experiment. But, instead of preparing the piece of marble by a 

 long continued heat, it will be sufficient to give it a coat of copal varnish, or 

 amber, or lac varnish : after which it must be kept in an oven for a short time. 

 By this means even the worst sort of marble answers very well, even without 

 previously warming or keeping it hot during the experiment. By means of the 

 varnish even a metal plate may be used instead of the marble. This should be 

 first made flat by grinding it against the upper plate, and then it must be var- 

 nished, but rather thicker than when the varnish is laid on marble. In this case 

 both the plates might be varnished, though it is sufficient to varnish one of 



them. 



The advantages which a varnished plate has above the common electrophorus, 

 are 1. That the varnish is always thinner than the common resinous stratum of 

 an electrophorus. 2. That the varnish acquires a more smooth and plain sur- 

 face ; hence the metal plate may be more easily, and to more advantage adapted 

 to it. Instead of the above-mentioned plane of marble or metal varnished, there 

 may be substituted, with equal advantage, any sort of plane covered with dry and 

 clean oil cloth or oil silk or sattin, or other silk stuff that is not very thick ; 

 which will answer very well, without requiring any more than perhaps a slight 

 warming. The silk stuffs answer better for this purpose than those made of 

 cotton or wool, and these better than linen. However, by a previous drying, 

 and keeping them hot during the experiment, paper, leather, wood, ivory, bone, 

 and every sort of imperfect conductor, may be made to answer to a certain de- 



