xii On the Method of rendering very fenfible 
tain meafure only, the free paflage of the eleClric fluid through 
its fubftance. There are many conductors of this kind ; as, 
for inftance, a clean and dry marble flab, a plate of wood (like- 
wife clean and very dry, or covered with a coat of varnifh, or 
wax) and the like. The furface of thole bodies does not con- 
tract any electricity, or if any electricity adheres to them, it 
vanilhes loon, on account of their femi- conducting nature; for 
which realon they cannot anfwer the office of an eleCtrophorus* 
and therefore are more fit to be ufed as condenfers of eleCtricitv. 
12. Befides the advantages above mentioned, there is ano- 
ther, which arifes from fubftituting an imperfeCtly conducting 
plane to the refinous plate, namely, that the metal plate laid 
upon one of thefe does actually condenfe or acquire a greater 
quantity of eleCtricity than when laid upon the refinous plate, 
or other perfeCt eleCtric ; for fince, as was find above, § 6. 
the thinner the refinous lfratum is, the better it anfwers our 
purpofe ; in the cafe of a varnifhed or waxed board, this ftra- 
tum becomes exceedingly thin, and it becomes nothing when 
an imperfeCtly conduding fubftance is ufed, fuch as a marble 
flab, a very dry piece of wood, &c. 
13. On the other hand, care fhould be taken, in choofing the 
above mentioned plane, that it be not too much of a conducting 
nature, or capable of becoming fo in a very fhort time, it 
being quite neceflary, that the eleCtricity fhould find a confi- 
derable degree of refiftance in going through its fubftance. In 
choofing, or in preparing, fuch a plane by drying, or other- 
wife, it is better to render it too near to than too far from 
the nature of a non-conduCtor. A marble flab, or a board, 
properly dried, anfwers admirably well, and is preferable to 
any other plane : otherwile the refinous plate of an ele&ro- 
phorus is preferable to a common table or marble flab not pre- 
pared ; for thefe bodies, being in fome meafure imbibed with 
moifture, conduCt much better than is neceflary. 
14. To be more particular, I fhall add, that for this pur- 
pole it is better to ufe a flat piece of marble, and to grind 
it againft the metal plate, till they coincide fo well as to 
fhew a fenfible cohelion between them. Afterwards the 
piece of marble fhould be expofed for feveral days to the 
heat 
