It is a long Time since I had the Pleasure of a Line from you. And indeed the Troubles of our Country, with the Hurry of Business, I have been engag’d in on that Account, have made me so bad a Correspondent, that I ought not to expect Punctuality in others.

But being just taking Passage for England, I could not leave the Continent,2 without paying my Respects to you, and at the same Time taking Leave to introduce to your Acquaintance a Gentleman of Learning and Merit, Col. Henry Bouquet,3 who does me the Favour to present you this Letter, and with whom I am sure you will be much pleased.

Mr. Professor Simpson of Glasgow, lately communicated to me some curious Experiments of a Physician of his Acquaintance, by which it appeared that an extraordinary Degree of Cold, even to Freezing, might be produced by Evaporation.4 I have not had Leisure to repeat and examine more than the first and easiest of them, viz. Wet the Ball of a Thermometer by a Feather dipt in Spirit of Wine, which has been kept in the same Room, and has of Course the same Degree of Heat or Cold. The Mercury sinks presently 3 or 4 Degrees, and the quicker if during the Evaporation you blow on the Ball with Bellows; a second Wetting and Blowing when the Mercury is down, carries it yet lower. I think I did not get it lower than 5 or 6 Degrees from where it naturally stood, which was at that time 60. But it is said, that a Vessel of Water being plac’d in another somewhat larger containing Spirit, in such a Manner that the Vessel of Water is surrounded with the Spirit, and both plac’d under the Receiver of an Air-pump, on Exhausting the Air, the Spirit evaporating leaves such a Degree of Cold as to freeze the Water, tho’ the Thermometer in the open Air stands many Degrees above the Freezing Point.

I know not how this Phenomenon is to be accounted for, but it gives me Occasion to mention some loose Notions relating to Heat and Cold, which I have for some Time entertain’d, but not yet reduc’d into any Form. Allowing common Fire as well as the Electrical, to be a Fluid, capable of permeating other Bodies, and seeking an Equilibrium, I imagine some Bodies are better fitted by Nature to be Conductors of that Fluid than others; and that generally those which are the best Conductors of the Electrical Fluid, are also the best Conductors of this; and e contra. Thus a Body which is a good Conductor of Fire readily receives it into its Substance, and conducts it thro’ the Whole to all the Parts; as Metals and Water do; and if two Bodies, both good Conductors, one heated, the other in its common State, are brought into Contact with each other, the Body which has most Fire, readily communicates of it to that which had least; and that which had least readily receives it, till an Equilibrium is produced. Thus, if you take a Dollar between your Fingers with one Hand, and a Piece of Wood of the same Dimensions with the other, and bring both at the same Time to the Flame of a Candle, you will find yourself obliged to drop the Dollar before you drop the Wood, because it conducts the Heat of the Candle sooner to your Flesh. Thus, if a Silver Teapot had a Handle of the same Metal, it would conduct the Heat from the Water to the Hand, and become too hot to be used; we therefore give to a Metal Teapot a Handle of Wood, which is not so good a Conductor as Metal. But a China or Stone Teapot being in some Degree of the Nature of Glass, which is not a good Conductor of Heat, may have a Handle of the same Stuff. Thus also a damp moist Air shall make a Man more sensible of Cold, or chill him more than a dry Air that is colder, because a moist Air is fitter to receive and conduct away the Heat of his Body. This Fluid entring Bodies in great Quantity, first expands them by separating their Parts a little, afterwards by farther separating their Parts, it renders solids fluid, and at length dissipates their Parts in Air. Take this Fluid from melted Lead, or from Water, the Parts cohere again, the first grows solid, the latter becomes Ice. And this is soonest done by the Means of good Conductors. Thus, if you take (as I have done) a square Bar of Lead, 4 Inches long, and 1 Inch thick, together with 3 Pieces of Wood planed to the same Dimensions, and lay them as in the Margin,

on a smooth Board, fix’d so as not to be easily separated or moved, and pour into the square5 Cavity they form as much melted Lead as will fill it, you will see the melted Lead chill and become firm on the Side next the Leaden Bar, some Time before it chills on the other three Sides in Contact with the Wooden Bars; tho’ before the Lead was poured in, they might all be supposed to have the same Degree of Heat or Coldness, as they had been exposed in the same Room to the same Air. You will likewise observe, that the leaden Bar, as it has cooled the melted Lead more than the wooden Bars have done, so it is itself more heated by the melted Lead. There is a certain Quantity of this Fluid, called Fire, in every living human Body, which Fluid, being in due Proportion, keeps the Parts of the Flesh and Blood at such a just Distance from each other, as that the Flesh and Nerves are suple, and the Blood fit for Circulation. If Part of this due Proportion of Fire be conducted away by Means of a Contact with other Bodies, as Air, Water, or Metals, the Parts of our Skin and Flesh that come into such Contact, first draw more near together than is agreeable, and give that Sensation which we call Cold, and if too much be conveyed away, the Body stiffens, the Blood ceases to flow, and Death ensues. On the other Hand, if too much of this Fluid be communicated to the Flesh, the Parts are separated too far, and Pain ensues as when they are separated by a Pin or Lancet. The Sensation that the Separation by Fire occasions, we call Heat, or Burning. My Desk, on which I now write, and the Lock of my Desk, are both expos’d to the same Temperature of the Air, and have therefore the same Degree of Heat and Cold; yet if I lay my Hand successively on the Wood and on the Metal, the latter feels much the Coldest; not that it is really so, but being a better Conductor, it more readily than the Wood takes away and draws into it self the Fire that was in my Skin. Accordingly, if I lay one Hand, Part on the Lock, and Part on the Wood, and after it has lain so some Time I feel both Parts with my other Hand, I find the Part that has been in Contact with the Lock, very sensibly colder to the Touch than the Part that lay on the Wood. How a living Animal obtains its Quantity of this Fluid called Fire, is a curious Question. I have shown that some Bodies (as Metals) have a Power of Attracting it stronger than others, and I have sometimes suspected that a living Body had some Power of Attracting out of the Air or other Bodies the Heat it wanted. Thus Metal hammer’d or repeatedly bent, grows hot in the bent or hammered Part. But when I consider’d that Air in contact with the Body cools it; that the surrounding Air is rather heated by its Contact with the Body; that every Breath of cooler Air drawn in, carries off Part of the Body’s Heat when it passes out again: That therefore there must be in the Body a Fund for producing it, or otherwise the Animal would soon grow cold: I have been rather enclin’d to think that the Fluid, Fire, as well as the Fluid, Air, is attracted by Plants in their Growth, and becomes consolidated with the other Materials of which they are formed, and makes a great Part of their Substance. That when they come to be digested, and to suffer in the Vessels a Kind of Fermentation, Part of the Fire as well as Part of the Air, recovers its fluid Active State again, and diffuses itself in the Body digesting and separating it. That the Fire so reproduc’d by Digestion and Separation, continually leaving the Body, its Place is supply’d by fresh Quantities arising from the continual Separation. That whatever quickens the Motion of the Fluids in an Animal, quickens the Separation, and reproduces more of the Fire, as Exercise. That all the Fire emitted by Wood and other Combustibles when burning, existed in them before in a solid State, being only discovered when separating. That some Fossils, as Sulphur, Seacoal, &c. contain a great deal of solid Fire; that Gunpowder is almost all solid Fire:6 And that, in short, what escapes and is dissipated in the Burning of Bodies, besides Water and Earth, is generally the Air and Fire that before made Parts of the solid. Thus I imagin that Animal Heat arises by or from a Kind of Fermentation in the Juices of the Body, in the same Manner as Heat arises in the Liquors preparing for Distillation; wherein there is a Separation of the spirituous from the watry and earthy Parts. And it is remarkable, that the Liquor in a Distiller’s Vat, when in its highest and best State of Fermentation, shows by the Thermometer,7 as I have been informed, the same Degree of Heat with the human Body, that is about 94 or 96. Thus, as by a constant Supply of Fuel in a Chimney, you keep a warm Room, so by a constant Supply of Food in the Stomach, you keep a warm Body. Only where little Exercise is used, the Heat may possibly be conducted away too fast, in which Case such Materials are to be used for Cloathing and Bedding, against the Effect of an immediate Contact of the Air, as are in themselves bad Conductors of Heat, and consequently prevent its being communicated thro’ their Substance to the Air. Hence what is called Warmth in Wool, and its Preference on that Account to Linen; Wool not being so good a Conductor. And hence all the natural Coverings of Animals to keep them warm, are such, as retain and confine the natural Heat in the Body, by being bad Conductors; such as Wool, Hair, Feathers, and the Silk by which the Silk-worm in its tender embrio State is first cloathed. Cloathing, thus considered, does not make a Man warm, by giving Warmth, but by preventing the too quick Dissipation of the Heat produc’d in his Body, and so occasioning an Accumulation.

There is another curious Question I will just venture to touch upon, viz. Whence arises the sudden extraordinary Degree of Cold, perceptible on mixing some Chymical Liquors, and even on mixing Salt and Snow, where the Composition appears colder than the coldest of the Ingredients? I have never seen the chymical Mixtures made, but Salt and Snow I have often mixed myself, and am fully satisfied that the Composition feels much colder to the Touch, and lowers the Mercury in the Thermometer more than either Ingredient would do separately. I suppose with others, that Cold is nothing more than an Absence of Heat or Fire. Now if the Quantity of Fire before contain’d or diffus’d in the Snow and Salt, was expell’d in the Uniting of the two Matters, it must be driven away either thro’ the Air or the Vessel containing them. If it is driven off thro’ the Air, it must warm the Air, and a Thermometer held over the Mixture without touching it, would discover the Heat by the Rising of the Mercury, as it must and always does in warmer Air. This indeed I have not try’d; but I should guess it would rather be driven off thro’ the Vessel, especially if the Vessel be Metal, as being a better Conductor than Air, and so one should find the Bason warmer after such Mixture. But on the contrary the Vessel grows cold, and even Water in which the Vessel is sometimes plac’d for the Experiment, freezes into hard Ice on the Bason. Now I know not how to account for this otherwise than by supposing, that the Composition is a better Conductor of Fire than the Ingredients separately, and like the Lock compar’d with the Wood, has a stronger Power of Attracting Fire, and does accordingly attract it suddenly from the Fingers or a Thermometer put into it, from the Bason that contains it, and from the Water in contact with the Outside of the Bason, so that the Fingers have the Sensation of extream Cold, by being depriv’d of much of their natural Fire; the Thermometer sinks, by having part of its Fire drawn out of the Mercury; the Bason grows colder to the Touch, as by having its Fire drawn into the Mixture, it is become more capable of drawing and receiving it from the Hand; and thro’ the Bason the Water loses its Fire that kept it fluid, so it becomes Ice. One would expect, That from all this attracted Acquisition of Fire to the Composition, it should become warmer; and in fact, the Snow and Salt dissolves at the same Time into Water without freezing.8

I doubt whether in all this I have talked intelligibly; and indeed how should a Man do so, that does not himself clearly understand the Thing he talks of. This I confess to be my present Case. I intended to amuse you, but I fear I have done more, and tired you. Be so good as to excuse it, and believe me, with sincere Esteem and Respect, Sir, Your most obedient humble Servant