Your Favour of the 14th. Instant, I met on the Road in my Journey to this Place with my Son, who joins with me in Thanks for your good Wishes relating to his Administration here. I thank you also for your kind Endeavours in obtaining the Discharge of William Forrester, who is accordingly discharged.

I condole with you most sincerely on the repeated grievous Breaches that have been made in your Family. Loss of Friends and near and dear Relations, is one of the Taxes we pay for the Advantage of long Life, and a heavy Tax indeed it is!

The most remarkable Discovery that has been made within these three Years, is, that Quicksilver is in reality a melted Metal; with this new Character only, that of all others it requires the least Heat to melt it. The Academy of Sciences at Petersburgh, have found, that by dipping a mercurial Thermometer into repeated cooling Mixtures, and so taking from the Mercury the Heat that was in it, they have brought it down some hundred Degrees (the exact Number I cannot remember) below the Freezing Point, when the Mercury became solid, and would sink no lower; and then the Glass being broke, it came out in the Form of a silver Bullet, adhering to a Wire which was the slender part that had been in the Tube. Upon Tryal it was found malleable, and was hammer’d out to the Bigness of a Half Crown; but soon after, on receiving a small Degree of Warmth it return’d gradually to its fluid State again. This Experiment was repeated by several Members of that Academy two Winters successively, and an authentic Account of it transmitted to our Royal Society.

I suppose you have seen, in the 2d Vol. of the new Philosophical Essays of the Edinburgh Society, an Account of some Experiments to produce Cold by Evaporation, made by Dr. Cullen, who mentions the like having been before made at Petersburg. I think it is but lately that our European Philosophers have known or acknowledg’d anything of such a Power in Nature. But I find it has been long known in the East. Bernier, in the Account of his Travels into India, wrote above 100 Years since, mentions the Custom of Travellers carrying their Water in Flasks covered with wet woollen Cloth, and hung to the Pomells of their Saddles, so as that the Wind might act upon them in order to cool the Water. I have also seen a kind of Jar for cooling Water, made of Potter’s Earth, but unglaz’d and so porous that the Water gradually ouzed through to the Surface, supplying Water just sufficient for a constant Evaporation. I try’d it, and found the Water within much cool’d in a few Hours. This Jar was brought from Egypt. Mentioning this Matter to Dr. Hadley, then Professor of Chemistry at Cambridge, he immediately made the following Experiment at which I was present: He dipt the Thermometer into a Quantity of the highly rectified Spirit that goes by the Name of Ether, famous for its quick Evaporation. The Ether having stood in the same Room with the Thermometer, was therefore of the same degree of Warmth, and of course there was no Alteration of Height in the Mercury while it remain’d in the Ether. But as soon as the Thermometer was taken out, and the Ether with which it was wet began to evaporate from its Surface, the Mercury fell several Degrees. The Doctor repeated the Wetting with a Feather, and I quicken’d the Evaporation by a Bellows, that the Repetitions of Wetting might succeed quicker; and the Event was, that the Mercury sunk from 65, the Temperament of the Air on that Day, down to 7, which was 25 Degrees below the Freezing Point; so that the Moisture of the Air and Breath of the Bystanders, collecting round the cold Ball of the Thermometer, froze there, and form’d a rough Ice all round, and near ¼ of an Inch thick. It seems by this, that a Man naked, and standing in the Wind, and repeatedly wet with Spirits might be frozen to death in a Summer’s Day. I think none of our common Philosophical Principles will serve us in accounting for this. Perhaps yours may do it.

Professor Aepinus of Petersburgh, has publish’d a Work on Magnetism and Electricity, in which he endeavours to apply my Theory of the latter to the Explanation of certain Phenomena in the former. He supposes Magnetism to be a particular Fluid equally diffus’d in all Iron, easily flowing and easily moving in soft Iron, so as to maintain or recover an Equilibrium. But in hard Iron, i.e. Steel, moving with more Difficulty. And that in making an Artificial Magnet of Steel, nothing is added, the Magnetism is only moved from one End towards the other, and not easily returning remains there, so that one end of it has more of it than its natural Quantity, the other less; or in other Words, one End has positive Magnetism, the other negative. I have while in England made many Experiments with Magnets, and own myself inclined to Aepinus’s Opinion, tho’ there are some Difficulties in his Doctrine that as yet I do not see how to solve. Shall I venture to mention moreover, a Suspicion I have, to wit, that Magnetism fills all Space, and that its general Direction is what we call North and South, except only where it is made to deviate by Nearness to particular Magnets, as this Globe of Earth, a Stone, or Steel Bar; and that a Being capable of Passing from one Heavenly Body to another, might be directed in his Course by a Compass, as well as a Mariner on our Ocean. I suspect too, that one Use of this Universal Magnetism may be, to keep the Axes of the Planets and other heavenly Bodies nearly in their respective Directions; tho’ I at the same time see Difficulties in and Objections to this Opinion that are yet too hard for me.

While in England, after my chief Business was over, I amus’d myself, with contriving and bringing to a considerable Degree of Perfection, a new musical Instrument, which has afforded me and my Friends a great deal of Pleasure; but the Description of it would make this Letter, already too long, much longer. I am, with great Regard and Sincerity, Dear Sir, Your most obedient and most humble Servant