To Thomas Ronayne7
MS not found; reprinted from William Temple Franklin, ed., Memoirs of the Life and Writings of Benjamin Franklin, LL.D. F.R.S. &c. (Quarto edition, London, 1817–18), iii, 364–6.8
London, April 20, 1766.
I received your very obliging and ingenious letter by Captain Kearney.9 Your observations on the Electricity of Fogs, and of the air in Ireland, and of the several circumstances attending a thunder-storm, are very curious; and I thank you for them. I have endeavored to get Father Beccaria’s book for you, but find it is not to be had here: ’tis in 2 vols. 4to. in Italian, printed at Turin.1 In my opinion no part of the earth is, or can be, in a negative state of electricity naturally; and though an inequality may in some circumstances be occasioned, an equality would soon follow, from the extreme subtilty of the electric fluid, and the good conductors the moist earth is filled with. But yet I think when a highly-charged positive cloud comes near the earth, it repels and drives inward the natural quantity of electricity in the superficial parts, and in buildings, trees, &c. so as to bring them into a real negative state before it strikes. And I think the negative state you often find your balls in that hang to your apparatus, is not occasioned always by negative clouds, but often by positive clouds having passed over it, which, in passing, have repelled and driven out part of the natural quantity of electricity that was in the apparatus; so that when they are passed, the remainder diffusing itself equally in the apparatus, the whole is in a negative state. If you have read my experiments in pursuance of those made by Mr. Canton (they are in vol. 40 of the Transactions), you will easily understand this.2 But you may readily make some experiments that will show it clearly. Make a common wine-glass warm by the fire, that it may keep quite dry for some time; set it on a table, and place on it Mr. Canton’s little box, the balls hanging from the box a little beyond the edge of the table. Rub another warm wine-glass with a piece of black silk, or even a common silk handkerchief, so as to excite it. Then bring the glass over the box at the end farthest from the balls, at three or four inches’ distance, and you will see the balls diverge, being then electrified positively by the natural quantity of electricity that was in the box, driven to that end by the repelling force of the atmosphere of the rubbed glass. Touch the box near the balls (the rubbed glass remaining as at first), and the balls will come together, your finger taking away the quantity driven to that end. Then withdraw finger and glass at the same time, and the quantity left in the box diffusing itself equally, the balls will diverge again and be negative. While in this state, rub your glass afresh, and pass it over the box without coming too near, and you will see, as you approach it, the balls first close; they are then in a natural state: as the glass comes nearer, they open again; they are then positive. When the glass passes and begins to leave them, they close again, and are then in the natural state. When it has quite left them, they open again, and are then negative. The rubbed glass may represent a positively charged cloud, which you see is thus capable of producing all the changes in the apparatus without the necessity of supposing any negative cloud at all. But yet I am convinced there are negative clouds; because they will sometimes drink at and through the apparatus a large full bottle of positive electricity, of which the apparatus itself could not have received and retained the hundredth part. And, indeed, it is easy to conceive how a strongly charged large positive cloud may reduce smaller clouds to a negative state, as it passes over or near them, by driving their natural quantity out of them to their under side, whence it strikes into the earth, or to their farther end, whence it strikes into neighboring clouds; so that when the great cloud has passed or removed farther, they are left in a negative state, like the apparatus; they being, as well as it, often insulated bodies, not in contact with the earth or one another. And in the same manner it is equally easy to conceive how a large negative cloud may make others positive. The experiment you mention of filing the glass is similar to one I made in 1751, or 1752.3 I had supposed in my letter4 that the internal pores of glass were less than those near the surface, and so denied a passage to the electric fluid. To try whether this was so in fact, I ground one of my phials on one side extremely thin, passing a good way beyond the middle of the thickness, and very near to the other side, as I found on breaking it after the experiment. It charged as well after grinding as before; which satisfied me that my hypothesis was in that particular wrong. It is hard to conceive where the additional quantity on the charged side of glass is deposited, there is so much of it. I send you my meteorological paper, which has lately been printed here in the Transactions,5 following a paper of Mr. Hamilton’s on the same subject.6 I am, &c.
7. For Thomas Ronayne, an Irishman whose interest was “atmospheric Electricity” and who had corresponded with BF on the subject as early as 1761, see above, IX, 350–2.
8. A French translation of this letter was printed earlier in Jacques Barbeu Dubourg, ed., Oeuvres de M. Franklin, Docteur ès Lois (Paris, 1773), i, 265–8. William Temple Franklin probably printed from a retained copy of the letter, though he may possibly have made a retranslation from Dubourg’s French text.
9. Ronayne’s letter has not been found.
1. BF did manage, however, to obtain a copy of Beccaria’s Dell’ elettricismo artificiale e naturale for Joseph Priestley who requested it in his letter of April 13, 1766. See above, p. 246 n.
2. The experiments to which BF here refers were published in volume 49 of Phil. Trans. See above, V, 516–19. Dubourg’s translation omits the words in parentheses, substituting a footnote cross reference to his own edition.
3. BF described this experiment in a letter to Dr. John Lining, March 18, 1755; see above, V, 522–3.
4. Presumably his “Opinions and Conjectures,” [July 29, 1750]. See above, IV, 25–34.
5. See above, IV, 235–43.
6. Hugh Hamilton (1729–1805), professor of natural philosophy at the University of Dublin, was a cleric and a writer on scientific subjects. DNB. His “A Dissertation on the Nature of Evaporation and several Phaenomena of Air, Water, and boiling Liquors” was read before the Royal Society, May 16, 1765, and published in Phil. Trans., lv (1765), 146–81.