From John Canton6
ALS: The Royal Society7
Spital Square, 31 Octr. 1760
Having procur’d some thin Glass Balls of about an Inch and a half in Diameter, with Stems, or Tubes of eight or nine Inches in length, I electrified them, some positively on the inside, and others negatively, after the manner of charging the Leyden Bottle, and sealed them hermetically. Soon after, I applied the naked Balls to my Electrometer, and could not discover the least Sign of their being electrical; but holding them before the Fire at the distance of six or eight Inches, they became strongly electrical in a very short time, and more so, when they were cooling.8 These Balls will, every time they are heated, give the electrical Fluid to, or take it from other Bodies, according to the plus or minus State of it within them. Heating them frequently, I find, will sensibly diminish their Power; but keeping one of them under Water a Week, did not appear in the least degree to impair it. That which I kept under Water was charged on the 22d of September last, was several times heated before it was kept in Water and has been heated frequently since, and yet it still retains its Virtue to a very considerable degree. The breaking two of my Balls accidentally, gave me an Opportunity of measuring their Thickness, which I found to be between seven and eight parts in a Thousand, of an Inch.
A down Feather in a thin Glass Ball hermetically sealed, will not be affected by the Application of an excited Tube, or the Wire of a charged Vial, unless the Ball be considerably heated. And if a Glass Pane be heated till it begins to grow soft, and in that State be held between the Wire of a Charged Vial, and the discharging Wire, the Course of the electrical Fluid will not be through the Glass, but on the Surface, round by the Edge of it. I am, with the greatest Respect, Dear Sir Your most Obedient humble Servant
To Benjn. Franklin Esqr.
6. On John Canton, electrical experimenter, see above, IV, 390 n.
7. The MS, surviving among the Canton Papers, appears to be a retained copy. Franklin quoted the letter in full (omitting only the address, salutation, and complimentary close) in a letter to Ebenezer Kinnersley, Feb. 20, 1762, printed in the 1769 edition of Exper. and Obser., where the quotation appears on pp. 401–2.
8. The effect of heat on the permeability of glass by electricity was at this time attracting the attention of several experimenters.