To William Heberden8
ALS: Harvard College Library9
Tourmaline crystals, brought to Europe from the East by the Dutch early in the eighteenth century, began to attract the attention of electrical scientists when they found that, if heated, they had the power of attracting and repelling ashes and other light substances. The German scientist Aepinus began to study this pyroelectrical property and wrote a paper in 1756, in which he reported that, when a tourmaline was heated in boiling water, one side was electrified positively and the opposite side negatively, and that these charges could also be affected in various ways by other experimental treatments.1 An Italian investigator, Giovanni Carafa, Duke of Noja, disputed these findings in a paper published in 1759.2 Franklin’s friend Dr. Heberden became interested in the matter, procured a set of tourmalines, and lent them to various English acquaintances for experimental purposes. Franklin’s letter is the earliest known report on such investigations in England. Since it was not published for ten years, however, or read to the Royal Society, his friends John Canton and Benjamin Wilson, accounts of whose experiments became public later in 1759, gained general priority among English investigators of the subject.3
Thursday June 7. ’59.
I have been so long in determining which to chuse of the two Tourmalines, that I fear you begin to think me unreasonable enough to keep them both. I now return the small one, and beg your Acceptance of my sincerest Thanks for the other, which tho’ I value it highly for its rare and wonderful Properties, I shall ever esteem it more for the Friendship I am honour’d with by the Giver.
I hear that the Negative Electricity of one Side of the Tourmalin when heated, is absolutely denied, and all that has been related of it ascrib’d to Prejudice in favour of a System, by some ingenious Gentlemen abroad, who profess to have made Experiments on the Stone with Care and Exactness.4 The Experiments have succeeded differently with me; yet I would not call the Accuracy of those Gentlemen in question. Possibly the Tourmalins they have try’d were not properly cut; so that the positive and negative Powers were obliquely plac’d, or in some manner whereby their Effects were confus’d, or the negative Part more easily supply’d by the positive. Perhaps the Lapidaries who have hitherto cut these Stones, had no Regard to the Situation of the two Powers, but chose to make the Faces of the Stone where they could obtain the greatest Breadth or some other Advantage in the Form. If any of these Stones in their natural State can be procur’d, I think it would be right to endeavour finding, before they are cut, the two Sides that contain the opposite Powers, and make the Faces there. Possibly in that Case the Effects might be stronger and more distinct. For tho’ both these Stones that I have examin’d have evidently the two Properties, yet without the full Heat given by boiling Water they are somewhat confus’d; the Virtue seems strongest towards one End of the Face, and in the Middle or near the other End scarce discernible; and the Negative I find always weaker than the Positive.
I have had the large one new cut, so as to make both Sides alike; and find the Change of Form has made no Change of Power, but the Properties of each Side remain the same as I found them before. It is now set in a Ring in such a manner as to turn on an Axis that I may conveniently in making Experiments come at both Sides of the Stone. The little Rim of Gold it is set in has made no Alteration in its Effects. The Warmth of my Finger when I wear it, is sufficient to give it some Degree of Electricity, so that ’tis always ready to attract light Bodies.
The following Experiments have satisfy’d me, that M. Epinus’s Account of the positive and negative States of the opposite Sides of the heated Tourmalin, is well founded.
I heated the large Stone in boiling Water.
As soon as it was dry I brought it near a very small Cork Ball that was suspended by a silk Thread.
The Ball was attracted by one Face of the Stone, which I call A, and then repell’d.
The Ball in that State was also repell’d by the positively charg’d Wire of a Phial; and attracted by the other Side of the Stone B.
The Stone being fresh heated, and the Ball attracted by the Side B, was presently after repell’d by that Side.
In this second State it was repell’d by the negatively-charg’d Wire of a Phial.
Therefore, if the Principles now generally receiv’d, relating to Positive and Negative Electricity are true, the Side A of the large Stone, when the Stone is heated in Water, is in a positive State of Electricity, and the Side B in a negative State.
The same Experiments being made with the small Stone stuck by one Edge on the End of a small Glass Tube with Sealing Wax, the same Effects are produced.
The flat Side of the small Stone gives the Signs of positive Electricity; the high Side gives the Signs of negative Electricity.
I suspended the small Stone by a Silk Thread.
I heated it as it hung, in boiling Water.
I heated the large one in boiling Water.
Then I brought the large Stone near to the suspended small one; which immediately turn’d its flat Side to the Side B of the large Stone, and would cling to it.
I turn’d the Ring, so as to present the Side A of the large Stone to the flat Side of the small one.
The flat Side was repell’d, and the small Stone turning quick, apply’d its high Side to the Side A of the large one.
This was precisely what ought to happen on the Supposition that the flat Side of the small Stone, when heated in Water, is positive, and the high Side negative; the Side A of the large Stone positive, and the Side B negative.
I find that the Face A of the large Stone being coated with Leaf Gold (attach’d by the White of an Egg, which will bear dipping in hot Water) becomes quicker and stronger in its Effect on the Cork Ball, repelling it the Instant it comes in contact; which I suppose to be occasion’d by the united Force of different Parts of the Face collected and acting together thro’ the Metal.
You have a Right to all the Experiments and Observations I have made or may make on this admirable Stone.5 I therefore offer no Apology as giving you Trouble, when I only intend performing a Duty, which you may forbid whenever it becomes disagreable. With the greatest Esteem and Respect, I am, Dear Sir Your most obedient and most humble Servant
8. See above, p. 281 n.
9. Printed as Letter XXXIV in Exper. and Obser., 1769 edit., pp. 375–8, and 1774 edit., pp. 383–6.
1. “Mémoire concernant quelques nouvelles expériences éctriques remarquables,” Histoire de l’Académie Royale des Sciences et Belles Lettres. Année MDCCLVI (Berlin, 1758), pp. 105–21. An English summary was printed in Gent. Mag., XXVIII (1758 supplement), 617–19. Franz Ulrich Theodor Aepinus (1724–1802) was born in Saxony, studied medicine and the physical sciences, was admitted to the Academy of Sciences in Berlin, and in 1757 settled in St. Petersburg, where he was made a member of the Imperial Academy of Sciences and professor of physics. In the course of his electrical work Aepinus introduced important modifications of BF’s theories, which he nevertheless basically upheld.
2. Lettre du duc de Noya-Caraffa sur la tourmaline à Monsieur de Buffon (Paris, 1759).
3. Gent. Mag., XXIX (1759), 424–5; Phil. Trans., LI, pt. I (1759), 308–39. The fullest eighteenth-century account of this subject in English is in Joseph Priestley, The History and Present State of Electricity (London, 1767), pp. 314–26, 697–713. The second passage contains Priestley’s account of his own experiments, made in 1766 with Heberden’s tourmalines.
4. The Duke de Noja had asserted that, when heated, both sides of the tourmaline were positively charged, but one side more than the other.
5. BF is not known to have made further experiments with the tourmaline. Probably he felt that he could safely leave the matter in the hands of Canton and Wilson, who also confirmed Aepinus’ findings.