. On the Permeability of Glafs to Eleétricity. 149 
If it be required to obtain the acid perfeétly pure, nothing 
is neceffary but to feparate from it the fpirit of wine, which 
can be beft effected by evaporation. The acid of the lemons 
affumes, after it has been freed from the fpirit of wine and 
the moifture combined with it, a yellowifh colour, and be- 
comes fo ftrong that, by its tafte, it might be confidered as a 
mineral acid. 
It is not neceflary to evaporate the fpirit of wine ina clofé 
veffel, if the experiment is made only on a fmall feale; nor 
is there any danger that in open veffels any of the acid wilf- 
be loft, as it is too fixed to be volatilifed by the fame degree 
of heat at which fpirit of wine evaporates. This acid has 
peculiar properties, which deferve farther examination. 
VII. An Experiment to prove the Permeability of Glafs to the 
Eleétric Fluid. By Mr. W. Woop, F.L.S. Communi- 
cated by the Author, 
\ V HETHER eglafs is or is not permeable to the elec- 
- trie fluid has been Jong difputed among philofophers, and 
various have been the experiments made both-to prove and 
to refute the Franklinean hypothefis. 
A paper by Mr. Wilfon was read at the Royal Society, 
December 6, 1759, in favour of the permeability of glafs, 
in which the following experiments were related: He took 
avery large pane of glafs, a little warmed ; and holding it 
upright by one edge, while the oppofite edge refted upon 
wax, he rubbed the middle part of the furface with his 
finger, and found both fides eleétrified plus. 
He then held the fame pane of glafs within two feet of 
‘the prime conductor, which was eleétrified plus; that part of 
ithe glafs which was oppofite to the conductor became elec- 
trified minus on both fides; but, in a few minutes, the mi- 
nus electricity difappeared,.and the plus continuing, diffufed 
L3 itfelf 
