Franklin: Answers to Queries from Dr. Ingenhousz
AL (draft) and copy: Library of Congress
[after May 3, 1780]
By the Circumstances that have appear’d to me, in all the Jarrs that I have seen perforated at the Time of their Explosion, I have imagined that the Charge did not pass by those Perforations. Several single Jarrs that have broke while I was charging them, have shown, besides the Perforation in the Body, a Trace on both sides the Neck, wherein the Polish of the Glass was taken off, the Breadth of a Straw; which prov’d that great Part at least, of the Charge, probably all, had passed over that Trace. I was once present at the Discharge of a Battery containing 30 Jarrs, of which 8 were perforated and spoilt at the Time of the Discharge, yet the Effect of the Charge on the Bodies upon which it was intended to operate, did not appear to be diminished. Another Time I was present when twelve out of Twenty Jarrs were broken at the Time of the Discharge, yet the Effect of the Charge which pass’d in the regular Circuit, was the same as it would have been if they had remained whole. Were those Perforations an Effect of the Charge within the Jarr forcing itself thro’ the Glass to get at the Outside, other Difficulties would arise and demand Explanation, 1. How it happens that in 8 Bottles and in 12 the Strength to bear a strong Charge should be so equal, that no one of them would break before the rest and thereby save his Fellows; but all should burst at the same Instant? 2. How it happens that they bear the Force of the great Charge till the Instant that an easier Means of Discharge is offered them, which they make use of, and yet the Fluid breaks thro’ at the same time? My Conjecture is, that there has been in the Place where the Rupture happens, some Defect in the Glass, some Grain of Sand perhaps, or some little Bubble in the Substance, nearly void, where during the Charging of the Jarr the Electric Fluid is forc’d in and confin’d, till the Pressure is suddenly taken off by the Discharge, when not being able to escape so quickly, it bursts its way out by its elastic Force. Hence all the Ruptures happen nearly at the same Instant with the regular Discharge, tho’ really a little posterior, not being themselves Discharges, but the Effects of a Discharge which pass’d in another Channel.
Those Impressions are not Effects of a moving Body, striking with Force in the Direction of its Motion; they are made by the Burs rising in the neighbouring perforated Cards, which rise accidentally sometimes on one Side of a Card, sometimes on the other in consequence of certain Circumstances in the Form of their Surfaces or Substances or Situations. In a single Card supported without touching others while perforated by the passing Fluid, the Bur generally rises on both Sides, as I once show’d to Mr Symmer at his House.1 I imagine that the Hole is made by a fine Thread of El. Fluid first passing, and augmented to a bigger Thread, at the Time of the Explosion, which obliging the Parts of the Card to recede every way, condenses a Part within the Substance, and forces a Part out on each side, because there is least Resistance.
This will be answer’d in my Remarks on M. Barletti’s Book which Remarks when finish’d I will send you.2
That the Electric Fluid by its repulsive Nature is capable of Forcing Portions of the same Fluid out of Bodies without entring them itself, appears from this Experiment. Approach an isolated Body with a rubb’d Tube of Glass; the Side next the Tube will then be electris’d negatively, the opposite positively. If a pair of Cork Balls hang from that opposite side, the Electrical Fluid forc’d out of the Body, will appear in those Balls, causing them to diverge. Touch that opposite Side, and you thereby take away the positive Electricity. Then Remove the Tube, and you leave the Body all in a negative State. Hence it appears that the Electric Fluid appertaining to the Glass Tube did not enter the Body; but retired with the Tube, otherwise it would have supply’d the Body with the Electricity it had lost.
With regard to Powder Magazines
My Idea is,
That to prevent the Mischief which might be occasion’d by the Stones of their Walls flying about in case of accidental Explosion, they should be constructed in the Ground; that the Walls should be lin’d with Lead, the Floor Lead, all ¼ Inch thick & the Joints well solder’d; the Cover Copper; with a little Scuttle to enter, the whole in the Form of a Canister for Tea. If the Edges of the Cover scuttle fall into a Copper Channel containing Mercury, not the smallest Particle of Air or Moisture can enter to the Powder, even tho’ the Walls stood in Water, or the whole was under Water.3
9. In the draft, BF inserted this title, and squeezed in Ingenhousz’s questions, after the answers had been written out. We print only his answers here, and refer readers to the preceding document for the questions.
1. For Robert Symmer see VIII, 417n; I. Bernard Cohen, Franklin and Newton … (Philadelphia, 1956), pp. 543–6; J.L. Heilbrun, Elements of Early Modern Physics (Berkeley, Los Angeles, and London, 1982), pp. 213–14.
2. BF finally sent them on June 21, 1782: “An Attempt to explain the Effects of Lightning on the Vane of the Steeple of a Church in Cremona, August, 1777. Addressed to Dr. John Ingenhouss.” The text is published in Smyth, Writings, VII, 88–97.
3. This is an entirely different conception than was tentatively proposed to the Académie des sciences in March, 1779, by a committee that included BF: XXIX, 209–11.