It should seem, that to make glass permeable to the electric fluid, the heat should be proportioned to the thickness. You found the heat of boiling water, which is but 210, sufficient to render the extreme thin glass in a Florence flask permeable even to a shock.—Lord Charles Cavendish, by a very ingenious experiment, has found the heat of 400 requisite to render thicker glass permeable to the common current.
"A glass tube, (See [Plate III].) of which the part C B was solid, had wire thrust in each end, reaching to B and C.
"A small wire was tied on at D, reaching to the floor, in order to carry off any electricity that might run along upon the tube.
"The bent part was placed in an iron pot, filled with iron filings; a thermometer was also put into the filings; a lamp was placed under the pot; and the whole was supported upon glass.
"The wire A being electrified by a machine, before the heat was applied, the corks at E separated, at first upon the principle of the Leyden phial.
"But after the part C B of the tube was heated to 600, the corks continued to separate, though you discharged the electricity by touching the wire at E, the electrical machine continuing in motion.
"Upon letting the whole cool, the effect remained till the thermometer was sunk to 400."
Vol. I. page 348.
Plate III.