the wcalicfl Natural or Artificial Il!c 61 r icily . xi i i 
Iient of o. warmed place, lucn as an. oven, a chimney, See. In 
order to expel the moifture, and to render it quite fit for our 
experiments (§ 12. 13.). The marble, thus prepared, will 
continue dry for a coniiderable time, except it be long expofed 
to very damp air. As for the frnall quantity of moifture w hich 
the marble may accidentally and fuperficially attradf, it may be 
removed by expofing it to the fun, or to a fire, or even by 
wiping it with a dry and clean cloth, previous to the per- 
forming of experiments. 
15. It is not every fort of marble that will anfwer 
equally for this purpole. The old marbles, which have 
been long preferved in dry places, anfwer better than thole 
which have been recently dug from the quarry. The dif- 
ference of the lpecies of marble is alio of confequence in this 
bufinefs ; I have found fome marbles which, without any 
preparation, anfwer vaflly well, whereas others will not do 
near lo well, even when properly prepared ; excepting, how- 
ever, when they are preferved hot during the experiment ; for, 
in that cafe, they anfwer better than the beft pieces of marble 
that are not preferved hot. It is always advantageous to warm 
the marble previous to the experiment. 
1 6- Inftead of preparing the piece of marble by a long con- 
tinued heat, it will be fufficient to give it a coat of copal var- 
liilh, or amber, or lac varnifh : after which it mull; be kept in 
an oven for a Ihort time. By this means even the worft fort of 
marble anfwers very well, even without previoufly warming 
or keeping it hot during the experiment. 
17. By means of the varnifh even a metal plate may be ufed 
in Bead of the marble. This Ihould be firft made flat by grind- 
ing it again!! the upper plate, and then it mull be varnilhed, but 
rather thicker than when the varnifh is laid upon* the marble. 
In this cafe both the plates might be varnilhed, though it is 
fufficient to varnifh one of them. 
18. Here it may be laid, that in fact we are returned to the ' 
eleCtrophorus. This is tine ; and indeed the varnilhed metal, 
or marble, or wood, may be excited by a very flight friction, 
even lbmetimes by the Ample laying of the metal plate upon it, 
C 2 efpecially 
