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not tinged more than a fourth part as much as when it was seen in its natural place through the same glass prism, included in another of plain water ; so that he had no doubt but that, if his glass prism had had a little less of the dispersive power, its errors would have been perfectly corrected; and, as it dispersed the rays very much, he was fatisfied that, if it had been made of crown glass, the colouring would even have lain the contrary way. The refracting angle of his glass prism was 58o; that of the plain water (which brought the'image into its natural place, and in which the glass prism was included) 46°, the contrary way; and that of the water impregnated with the face cbaruni, 36o. 30'

The opinion that light is not merely a phenomenon produced by the vibrations of a subtile medium, but that it confifts of small particles actually emitted from luminous bodies, has received great confirmation from the experiments that have been made with the Bolognian stone, and particularly from those made by Mr. Canton, with the phosphorus prepared by him, of which we have given a particular account in our 420 volume (June 1770, page 422, &c.] M. Homberg who, at the be, ginning of this century, maintained this opinion, seemed in deed to put the materiality of light, and its direct emanation from the sun, out of all reasonable doubt, and to have proved, by actual experiment, that, notwithstanding the tenuity or extreme minuteness of its particles, it acquired, by means of the amazing velocity with which it is projected from that luminary, a momentum sufficient to pue light substances in motion, and even fensibly to impel the end of a lever connected with the spring of a watch, on which he threw the solar rays collected into a focus by a burning glass. The accuracy of this experia ment, and the reality of this impulse, were however afterwards called in question, and various experiments, of a similar kind, were afterwards made by M. Mairan and M. du Foy; but from which nothing certain could be concluded : as there was reason to believe that the motions observed in their machinery mnight probably be produced by other causes, particularly by a current of air, heated and put in motion by the condensed solar rays that were thrown upon it.

The momentum of light was, fome years ago, atterr.pted to be afcertained, in a much more accurate manner than by the preceding inquirers, by Mr. Michell: and though his apparatus was disturbed by the experiment, and on other accounts he did not pursue it so far as he had intended, it was not wholly without success; and the conclufions which may be drawn from it,'Dr. Priestley observes, are curious and important.' We thall accordingly give au abridged description of the apparatus, and

of

of the trials that were made with it; an account of which was communicated to the Author by that gentleman.

The intire instrument weighed only about 10 grains, and consisted of a straight and very flender piece of wire, io inches tong, suspended like the needle of a mariner's compass, having a small agate cap fixed to the middle of it, which was supported by a fine pointed needle, on which it turned extremely freely in a horizontal direction. At one end of this wire was fixed, ia a vertical position, or at right angles to the plane of its motion, a kind of vane, consisting of a thin plate of copper, a little more than an inch square, against which the fun's rays, collected by a concave mirrour of about two feet diameter were to be di, rected, while the wire was kept in a horizontal situation by a fmall counterpoise at its other end. The instrument was in. clofed in an oblong box, the lid and front of which were of glass, and was thereby secured from any disturbance from the motion of the air : at the same time the long wirę was gently retained in a position parallel to the length of the box, by means of a magnet placed on the outside, acting on a small bit of sewing needle, previously rendered magnetical, and fixed to the horizontal wire.

The rays of the sun being thrown from the mirrour, and collected into a focus on the copper vane, it began to move, at about the rate of an inch in a second of time, till it had passed through a space of about two inches and an half, when it was stopped by the back of the box. • Themiirour being removed, the inftrument returned to its former situation, by means of the little needle and magnet; and the rays of the sun being then again thrown upon it, it again began to move, and struck against the back of the box as before ; and this was repeated three or four times, with the same success.'

Further to verify the experiment, the instrument was turned half round, so that the end to which the copperplate was fixed, and which had before lain towards the right hand, now lay towards the left; and the rays of the sun being again thrown upon the copperplate, it was again moved as before (but in a contrary dirc&ion) till it again ftruck against the back of the box. The experiment was repeated once or twice with the fame success. But by this time the form of the copperplate was so much altered by the extreme heat which it underwent, which brought it nearly to a state of fusion, and it accordingly varied so much from a vertical position, that it began to be affected in the same manner as the tail of a windmill : being now impelled by the stream of heated air, which moved upwards, with a force fufficient to drive it in opposition to the impulse of the rays of light. Particular circumstances prevented Mr. Michell from

prosccuting prosecuting this curious experiment any further ; but from these trials it may with the highest probability be inferred, that the motion of the wire was actually produced by the direct impulse of the solar rays against the copperplate.

Some philosophers have appeared to be under great concern. for the enormous expence of luminous matter incurred by the fun, by the continual emission of light in all directions. Dr. Priestley, however, from the data furnilhed by this experiment, calculates that the quantity of matter contained in the solar rays, that fell upon the abovementioned copperplate in a second of time, amounted to no more than the 1,200,000,000th part of a grain. He further finds that, from one square foot of the sun's surface, there issues only one 40,000th part of a grain of matter in a second ; that is, little more than two grains in a day, or about 670 pounds avoirdupois in 6000 years :-an expence of matter so small, that, in that time, it would have Mortened the sun's semidiameter no more than about 10 feet, if his body confifted of matter even of the density of water only.

We thall close our account of this performance, by presenting our Readers with a sketch of an ingenious and singular theory, relating to the more intimate nature of matter, which will probably be new to the generality of our English Readers, For though one of the two parents of this hypothesis is our. countryman, we do not believe that he ever communicated his notions on this subject to the public. On account of its nom velty and piquancy, or as the French fay, porer la bonne bouche, we have reserved this philosophical dainty to the last.

The easy solution of a great variety of phenomena respecting light, and particularly its ready transmission through traniparent bodies in almost all directions, together with some other confiderations, have suggested to cwo philosophers in different parts. of the world, this lingular system relating to the nature of mate ter; which, though it does not, like the hypothesis of Berkeley, absolutely expel all body out of the universe, robs it of one of its most substantial qualities—its impenetrability, the strongest. tenure certainly by which it holds its claim to existence in the popular belief; and reduces it seemingly to little more than a Aimfy phantom, having no other subAratum than certain phy. focal points, poffefred of powers; by the energy of which, and not by any direct contact, we acquire, according to this system, the notions we entertain of the solidity or impenetrability of body. M. Boscovich first published his notions on this subject in bis Theoria Philosophia Naturalis : but the fame hypothesis had likewise occurred to our Author's ingenious friend, Mr. Michell, in a very early part of his life, and without his having had any communication with M. Boscovich, or even knowing that there was such a person,

The

The system, in short, is this; that what we call matter is not a solid, impenetrablc fubftance, as has been perhaps universally taken for granted, but is an aggregate of physical points only, endued with powers of attraction and repulfion, taking place at different distances; that is, surrounded with va. rious spheres of attraction and repulsion, in the same manner as folid matter is generally supposed to be. If the degree of velocity therefore, or the momentum of any body in motion, be sufficiently great to overcome any of those powers of repulfion that it may meet with in any other body that opposes its paffage, it will find no difficulty in making its way through that body, and this even, without moving the particles of that other body cut of their place *: • for nothing, says the Author, will interfere, or penetrate one another, but powers, such as, we know, do, in fact, exist in the same place, and counterbaJance or over-rule one another; a circumstance which nerer had the appearance of a contradiction, or even of a difficulty.'

This doctrine of the mutual penetrability of matter may perhaps be reconciled to that lirong prejudice which stands moit in its way, by the following confiderations; where we have taken the opportunity, for the sake of some of our Readers, to enlarge a little on the ground-work of the Author.

It should seem to follow from hence that a cannon-ball, could it be projected with a sufficient velocity or momentum, might país through a fone-wall, without inaking a hole in it, or even displacing any of its particles. As however we know nothing more of this hypothesis than what we collect from Dr. Priestley's thort ketch of it here given, we may posibly, in this instance, have extended the proposition in the text too far. The principal dificulties that prevent our clear conception of this theory arise from the filence of the Author with regard to the nature, probable number, &c. of these pbsfical points in bodies :-as, Whether they are solid and impenetrable, &c.? We speak therefore at random, and under correction; but all that is meant by the Roman Professor and Mr. Michell, with respect to the mutual penetrability of matter, appears to us to be this; that the real quantity of matter, even in the most solid bodies, is inconceivably small; or, in other words, that the diameter of, or the space occupied by, each of the solid, indivisible particles that constitute body, is, as it were, infinitely small, compared with the distance between each of them. On this supposition, we can readily conceive that one cubic inch of gold may (on the application of an adequate force) be made to occupy the same identical cubic inch of space, already occupied by another cubic inch of the same matter ; without move ing in the leaił, or disordering the arrangement of, a singie particle of the former, or of its own. If even the most folid bodies are thus flimlily constituted, there can be little chance of their physical points interfering with, or impinging against each other.

As

As our idea of the impenetrability of maiter is in part acquired by the sensations that we experience in the handling of bodies, it may be sufficient, in order to facilitate the conception of this system, and pave the way to its reception, to Thew that, on the first touch, our sensations are, in general, fallacious, and that therefore they may likewise possibly continue to deceive us, on every successive augmentation of pressure or impulse, though carried even to the greatest possible degree.

When we press our finger lightly against a table, we have an idea of somewhat solid that resists it. Now, it has been rendered evident, by various optical and electrical experiments, that bodies which appear to be in contact (such as the links of a chain, for instance, suspended at one end, &c.) do not actually touch each other. Accordingly, the difficulty we at first meet with, in penetrating the table, or in forcing our finger into the same place with its external surface, does not arise, as we are apt to imagine, from any actual contact of the two fubstances, but from some power of repulfion near their surfaces, by which the finger is refifted by the table, though it does not

On increafing the pressure, and thereby overcoming, or getting within the sphere of, this first power of repulsion, (which is easily done) the philosopher, even, who is acquainted with the reality of this power, fancies that the finger is now impeded in its progress through the table, by the actual solidity of its parts. But the very same is the apprehension of the geherality of mankind with respect to the firf obstruction, • Why, therefore,' says the Author, applving this theory to the transmillion of light,' may not the next be only another {phere of repulsion, which may only require a greater force than we can apply to overcome it, without disordering the ara rangement of the constituent particles, but which may be overcome by a body moving with the amazing velocity of light?'

At the same time that M. Boscovich thews that many of the phenomena respecting this last-mentioned subtile substance are much more easily folved upon this hypothesis than any other ; he proves that it is by no means inconsistent with any thing that we know concerning the laws of mechanics, or our discoveries in natural philosophy. Still further to illustrate and to compleat our sketch of this curious system, we thall give, at full length, the Author's account of the train of ideas by which our countryman, Mr. Michell, was led, on his part, to form this scheme of the immateriality of matter, as it

may

be called, or rather of the mutual penetration of matter.' The thought first occurred to him on the reading Baxter's treatise on the Immatetiality of the Soul. He there found, lays Dr. Priestlev, thac

this Author's idea of matter was, that it conlisted, as it were, of bricks, cemented together by an immaterial mortar. These Rev. O&t, 1772.

Z

bricks,

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