avoid length in this letter, refer you to them. In wheels the particles of grease and oil acting as so many little rollers, and preventing friction between the wood and wood, do thereby prevent the collection of fire.

CONJECTURES ON THE NATURE OF FIRE.

[For the consideration of my dear Friend, David Rittenhause, Esq.] Universal space, as far as we know of it, seems to be filled with a subtil fluid, whose motion, or vibration, is called light.

This fluid may possibly be the same with that which, being attracted by and entering into another more solid matter, dilates the substance by separating the constituent particles, and so rendering some solid fluid, and maintaining the fluidity of others; of which fluid when our bodies are totally deprived, they are said to be frozen, when they have a proper quantity, they are in health and fit to perform all their functions; it is then called natural heat; when too much, it is fever; and when forced into the body in too great a proportion from without, it gives pain by separating and destroying the flesh, and is then called burning, and the fluid so entering and acting is called fire.

While organised bodies, animal or vegetable, are augmenting in growth, or are supplying their continual waste, is not this done by attracting and consolidating this fluid called fire, so as to form of it a part of their substance? and is it not a separation of the parts of such substance, which, dissolving its solid state, sets that subtil fluid at liberty, when it again makes its appearance as fire?

For the power of man relative to matter, seems limited to the dividing or to mixing the various kinds of it, or changing its form and appearance, by different compositions of it, but does not extend to the making or creating of new matter, or annihilating the old. Thus if fire be an original element or kind of matter, it is fixed and permanent in the universe; we cannot destroy any of it, or make addition to it, we can only separate it from that which confines it, and so set it at liberty, as when we put wood in a situation to be burnt, or transfer it from one solid to another; as when we make lime by burning stone, a part of the fire dislodged from the wood being left in the stone.

May not this fluid when at liberty be capable of penetrating and entering into all bodies, organised or not; quitting easily in totality those not organised,

VOL. III.

2 z

and quitting easily in part those which are, the part assumed and fixed remaining till the body is dissolved?

Is it not the fluid which keeps asunder the particles of air, permitting them to approach, or separating them more in proportion as its quantity is diminished or augmented?

Is it not the greater gravity of the particles of air, which forces the particles of this fluid to mount with the matters to which it is attached, as smoke or vapor?

Does it not seem to have a great affinity with water, since it will quit a solid to unite with that fluid, and go off with it in vapor, leaving the solid cold to the touch, and the degree measurable by the thermometer?

The vapor rises attached to this fluid; but at a certain height they separate, and the vapor descends in rain, retaining but little of it; in snow or hail less. What becomes of that fluid? Does it rise above our atmosphere, and mix with the universal mass of the same kind? or does a spherical stratum of it denser, or less mixed with air attracted by this globe and repelled or pushed up only to a certain height from its surface by the greater weight of air, remain there, surrounding the globe, and proceeding with it round the sun?

In such case, as there may be a continuity or communication of this fluid through the air quite down to the earth, is it not by the vibration given to it by the sun that light appears to us? and may it not be, that every one of the infinitely small vibrations, striking common matter with a certain force, enters its substance, is held there by attraction, and augmented by succeeding vibrations, till the matter has received as much as their force can drive into it?

Is it not thus that the surface of this globe is continually heated by such repeated vibrations in the day, and cooled by the escape of the heat when those vibrations are discontinued in the night, or intercepted and reflected by clouds?

Is it not thus that fire is amassed in, and makes the greatest part of, the substance of combustible bodies?

Perhaps when this globe was first formed, and its original particles took their place at certain, distances from the centre, in proportion to their greater or less gravity, the fluid fire attracted towards that centre might in great part be obliged, as lightest, to take place above the rest, and thus form the sphere of fire above supposed, which would afterwards be continually diminishing by the substance it afforded to organised bodies, and the quantity restored to it again by the burning or other separating of the parts of those bodies.

Is not the natural heat of animals thus produced, by separating in digestion the parts of food, and setting their fire at liberty?

Is it not this sphere of fire which kindles the wandering globes that sometimes pass through it in our course round the sun, have their surface kindled by it, and burst when their included air is greatly rarefied by the heat on their burning surfaces?

May it not have been from such considerations, that the ancient philosophers supposed a sphere of fire to exist above the air of our atmosphere?

CURIOUS INSTANCE OF THE EFFECT OF OIL ON WATER. TO DR. PRINGLE, LONDON.

SIR, Philadelphia, Dec. 1, 1762. During our passage to Madeira, the weather being warm, and the cabin windows constantly open, for the benefit of the air, the candles at night flared and ran.very much; which was an inconvenience. At Madeira we got oil to burn, and with a common glass tumbler or beaker, slung in wire, and suspended to the eieling of the cabin, and a little wire hoop for the wick, furnished with corks to float on the oil, I made an Italian lamp, that gave us very good light all over the table.-The glass at bottom contained water to about one third of its height; another third was taken up with oil; the rest was left empty, that the sides of the glass might protect the flame from the wind. There is nothing remarkable in all this; but what follows is particular. At supper, looking on the lamp, I remarked, that though the surface of the oil was perfectly tranquil, and duly preserved its position and distance with regard to the brim of the glass, the water under the oil was in great commotion, rising and falling in irregular waves, which continued during the whole evening. The lamp was kept burning as a watch-light all night, till the oil was spent, and the water only remained. In the morning I observed, that though the motion of the ship continued the same, the water was now quiet, and its surface as tranquil as that of the oil had been the evening before. At night again, when oil was put upon it, the water resumed its irregular motions, rising in high waves almost to the surface of the oil, but without disturbing the smooth level of that surface. And this was repeated every day during the voyage.

Since my arrival in America, I have repeated the experiment frequently thus: I have put a pack-thread round a tumbler, with strings of the same from each side, meeting above it in a knot at about a foot distance from the top of the tumbler. Then putting in as much water as would fill about one third part of the tumbler, I lifted it up by the knot, and swung it to and fro in the air; when the water appeared to keep its place in the tumbler as steadily as if it had been ice. But pouring gently in upon the water about as much oil, and then again swinging it in the air as before, the tranquillity before possessed by the water, was transferred to the surface of the oil, and the water under it was agitated with the same commotions as at sea.

I have shown this experiment to a number of ingenious persons. Those who are but slightly acquainted with the principles of hydrostatics, &c. are apt to fancy immediately that they understand it, and readily attempt to explain it ; but their explanations have been different, and to me not very intelligible. Others, more deeply skilled in those principles, seem to wonder at it, and promise to consider it. And I think it is worth considering: for a new appearance, if it cannot be explained by our old principles, may afford us new ones, of use perhaps in explaining some other obscure parts of natural knowledge. I am, &c. B. FRANKLIN.

ON THE ELECTRICITY OF FOGS.

TO THOMAS RONAYNE,-CORK.

London, April 20, 1766.

I received your very obliging and ingenious letter by Captain Kearney. Your observations on the Electricity of Fogs, and of the air in Ireland, and of the several circumstances attending a thunder-storm, are very curious; and I thank you for them. I have endeavored to get Father Beccaria's book for you, but find it is not to be had here: 'tis in 2 vols. 4to. in Italian, printed at Turin. In my opinion no part of the earth is, or can be, in a negative state of electricity naturally; and though an inequality may in some circumstances be occasioned, an equality would soon follow, from the extreme subtilty of the electric fluid, and the good conductors the moist earth is filled with. But yet I think when a highly-charged positive cloud comes near the earth, it repels and drives in

ward the natural quantity of electricity in the superficial parts, and in buildings, trees, &c. so as to bring them into a real negative state before it strikes. And I think the negative state you often find your balls in that hang to your apparatus, is not occasioned always by negative clouds, but often by positive clouds having passed over it, which, in passing, have repelled and driven out part of the natural quantity of electricity that was in the apparatus; so that when they are passed, the remainder diffusing itself equally in the apparatus, the whole is in a negative state. If you have read my experiments in pursuance of those made by Mr. Canton (they are in vol. 40 of the Transactions), you will easily understand this. But you may readily make some experiments that will show it clearly. Make a common wine-glass warm by the fire, that it may keep quite dry for some time; set it on a table, and place on it Mr. Canton's little box, the balls hanging from the box a little beyond the edge of the table. Rub another warm wine-glass with a piece of black silk, or even a common silk handkerchief, so as to excite it. Then bring the glass over the box at the end farthest from the balls, at three or four inches' distance, and you will see the balls diverge, being then electrified positively by the natural quantity of electricity that was in the box, driven to that end by the repelling force of the atmosphere of the rubbed glass. Touch the box near the balls (the rubbed glass remaining as at first), and the balls will come together, your finger taking away the quantity driven to that end. Then withdraw finger and glass at the same time, and the quantity left in the box diffusing itself equally, the balls will diverge again and be negative. While in this state, rub your glass afresh, and pass it over the box without coming too near, and you will see, as you approach it, the balls first close; they are then in a natural state: as the glass comes nearer, they open again; they are then positive. When the glass passes and begins to leave them, they close again, and are then in the natural state. When it has quite left them, they open again, and are then negative. The rubbed glass may represent a positively charged cloud, which you see is thus capable of producing all the changes in the apparatus without the necessity of supposing any negative cloud at all. But yet I am convinced there are negative clouds; because they will sometimes drink at and through the apparatus a large full bottle of positive electricity, of which the apparatus itself could not have received and retained the hundredth part. And, indeed, it is easy to conceive how a strongly charged large positive cloud may reduce smaller clouds to a negative state, as it passes over or near them, by driving their natural quantity out of them to their