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 know 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. 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. Farther,

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.

The Effect was apparently the same as would have been produc’d if one Magnet had been suspended by a Thread, and the different Poles of another brought alternately near it.

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. 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