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not tinged more than a fourth part as much as when it was feen in its natural place through the fame glafs prifm, included in another of plain water; fo that he had no doubt but that, if his glafs prifm had had a little lefs of the difperfive power, its errors would have been perfectly corrected; and, as it difperfed the rays very much, he was satisfied that, if it had been made of crown glafs, the colouring would even have lain the conThe refracting angle of his glafs prifm was 58°; trary way. that of the plain water (which brought the image into its natural place, and in which the glafs prifm was included) 46°, the contrary way; and that of the water impregnated with the fac charum, 36°. 30'.'

The opinion that light is not merely a phenomenon produced by the vibrations of a fubtile medium, but that it confists of fmall particles actually emitted from luminous bodies, has received great confirmation from the experiments that have been made with the Bolognian ftone, and particularly from thofe made by Mr. Canton, with the phofphorus prepared by him, of which we have given a particular account in our 42d volume [June 1770, page 422, &c.] M. Homberg who, at the beginning of this century, maintained this opinion, feemed indeed to put the materiality of light, and its direct emanation from the fun, out of all reasonable doubt, and to have proved, by actual experiment, that, notwithstanding the tenuity or extreme minutenefs of its particles, it acquired, by means of the amazing velocity with which it is projected from that luminary, a momentum fufficient to put light fubftances in motion, and even fenfibly to impel the end of a lever connected with the fpring of a watch, on which he threw the folar rays collected into a focus by a burning glafs. The accuracy of this experiment, and the reality of this impulfe, were however afterwards called in queftion, and various experiments, of a fimilar kind, were afterwards made by M. Mairan and M. du Foy; but from which nothing certain could be concluded: as there was reafon to believe that the motions obferved in their machinery might probably be produced by other caufes, particularly by a current of air, heated and put in motion by the condenfed folar rays that were thrown upon it.

The momentum of light was, fome years ago, attempted 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 purfue it fo far as he had intended, it was not wholly without fuccefs; and the conclufions which may be drawn from it, Dr. Priestley obferves, are curious and important.' We shall accordingly give an abridged defcription 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 inftrument weighed only about 10 grains, and confifted of a ftraight and very flender piece of wire, 10 inches tong, fufpended like the needle of a mariner's compass, having a fmall agate cap fixed to the middle of it, which was fupported by a fine pointed needle, on which it turned extremely freely in a horizontal direction. At one end of this wire was fixed, in a vertical pofition, or at right angles to the plane of its motion, a kind of vane, confifting of a thin plate of copper, a little more than an inch fquare, against which the fun's rays, collected by a concave mirrour of about two feet diameter were to be directed, while the wire was kept in a horizontal fituation by a fmall counterpoife at its other end. The inftrument was inclofed in an oblong box, the lid and front of which were of glafs, and was thereby fecured from any difturbance from the motion of the air: at the fame time the long wire was gently retained in a pofition parallel to the length of the box, by means of a magnet placed on the outfide, acting on a small bit of fewing needle, previously rendered magnetical, and fixed to the horizontal wire.

The rays of the fun 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 fecond of time, till it had passed through a space of about two inches and an half, when it was ftopped by the back of the box. The mirour being removed, the inftrument returned to its former fituation, by means of the little needle and magnet; and the rays of the fun being then again thrown upon it, it again began to move, and ftruck against the back of the box as before; and this was repeated three or four times, with the fame fuccefs.'

Further to verify the experiment, the inftrument was turned half round, fo 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 fun being again thrown upon the copperplate, it was again moved as before (but in a contrary direction) till it again ftruck against the back of the box. The experiment was repeated once or twice with the fame fuccefs. But by this time the form of the copperplate was fo much altered by the extreme heat which it underwent, which brought it nearly to a ftate of fufion, and it accordingly varied fo much from a vertical pofition, that it began to be affected in the fame manner as the fail of a windmill: being now impelled by the stream of heated air, which moved upwards, with a force fufficient to drive it in oppofition to the impulfe of the rays of light. Particular circumftances prevented Alr. Michell from

profecuting

profecuting 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 impulfe of the folar rays against the copperplate.

Some philofophers have appeared to be under great concern. for the enormous expence of luminous matter incurred by the fun, by the continual emiffion of light in all directions. Dr. Priestley, however, from the data furnished by this experiment, calculates that the quantity of matter contained in the folar 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 fun's furface, there iffues only one 40,000th part of a grain of matter in a fecond; that is, little more than two grains in a day, or about 670 pounds avoirdupois in 6000 years:-an ex-. pence of matter fo fmall, that, in that time, it would have fhortened the fun's femidiameter no more than about 10 feet, if his body confifted of matter even of the density of water only.

We fhall close our account of this performance, by prefenting our Readers with a fketch of an ingenious and fingular 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 hypothefis is our Countryman, we do not believe that he ever communicated his notions on this fubject to the public. On account of its novelty and piquancy, or as the French fay, pour la bonne bouche, we have referved this philofophical dainty to the last.

The eafy folution of a great variety of phenomena respecting light, and particularly its ready tranfmiffion through transparent bodies in almost all directions, together with fome other confiderations, have fuggefted to two philofophers in different parts. of the world, this fingular fyftem relating to the nature of matter; which, though it does not, like the hypothefis of Berkeley, abfolutely expel all body out of the univerfe, robs it of one of its moft fubftantial qualities-its impenetrability, the strongeft tenure certainly by which it holds its claim to existence in the popular belief; and reduces it feemingly to little more than a Alimfy phantom, having no other fubftratum than certain phyfical points, poffeffed of powers; by the energy of which, and not by any direct contact, we acquire, according to this fyftem, the notions we entertain of the folidity or impenetrability of body. M. Bofcovich firft publifhed his notions on this fubject in his Theoria Philofophie Naturalis: but the fame hypothefis had likewife 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. Bofcovich, or even knowing. that there was such a person,

The

The fyftem, in fhort, is this; that what we call matter is not a folid, impenetrable fubftance, as has been perhaps univerfally taken for granted, but is an aggregate of phyfical points only, endued with powers of attraction and repulfion, taking place at different diftances; that is, furrounded with va rious fpheres of attraction and repulfion, in the fame manner as folid matter is generally fuppofed to be.' If the degree of velocity therefore, or the momentum of any body in motion, be fufficiently great to overcome any of thofe powers of repulfion that it may meet with in any other body that oppofes 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, fays the Author, will interfere, or penetrate one another, but powers, fuch as, we know, do, in fact, exift in the fame place, and counterbalance or over-rule one another; a circumftance which never 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 frong prejudice which stands moit in' its way, by the following confiderations; where we have taken the opportunity, for the fake of fome of our Readers, to enlarge a little on the ground-work of the Author.

It should feem to follow from hence that a cannon-ball, could it be projected with a fufficient velocity or momentum, might país through a ftone-wall, without making a hole in it, or even difplacing any of its particles. As however we know nothing more of this hypothefis than what we collect from Dr. Priestley's thort sketch of it here given, we may poffibly, in this inftance, have extended the propofition in the text too far. The principal difficulties that prevent our clear conception of this theory arife from the filence of the Author with regard to the nature, probable number, &c. of these phyfical points in bodies:-as, Whether they are folid and impenetrable, &c.? We fpeak therefore at random, and under correction; but all that is meant by the Roman Profeffor 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 moft folid bodies, is inconceivably fmall; or, in other words, that the diameter of, or the space occupied by, each of the folid, indivifible particles that conflitute body, is, as it were, infinitely fmall, compared with the distance between each of them. On this fuppofition, we can readily conceive that one cubic inch of gold may (on the application of an adequate force) be made to occupy the fame identical cubic inch of space, already occupied by another cubic inch of the fame matter; without moving in the least, or difordering the arrangement of, a fingle particle of the former, or of its own. If even the moft folid bodies are thus flimfily 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 matter is in part acquired by the fenfations that we experience in the handling of bodies, it may be fuficient, in order to facilitate the conception of this fyftem, and pave the way to its reception, to fhew that, on the first touch, our fenfations are, in general, fallacious, and that therefore they may likewife poffibly continue to deceive us, on every fucceffive augmentation of preffure or impulfe, though carried even to the greateft poffible degree.

When we prefs our finger lightly against a table, we have an idea of fomewhat folid that refifts it. Now, it has been rendered evident, by various optical and electrical experiments, that bodies which appear to be in contact (fuch as the links of a chain, for instance, fufpended 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 fame place with its external furface, does not arife, as we are apt to imagine, from any actual contact of the two fubftances, but from fome power of repulfion near their furfaces, by which the finger is refifted by the table, though it does not touch it. On increafing the preffure, and thereby overcoming, or getting within the sphere of, this firft power of repulfion, (which is cafily done) the philofopher, 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 folidity of its parts. But the very fame is the apprehenfion of the ge herality of mankind with refpect to the first obstruction,

6

Why, therefore,' fays the Author, applying this theory to the tranfmiffion of light, may not the next be only another sphere of repulfion, which may only require a greater force than we can apply to overcome it, without difordering the arrangement of the conftituent particles, but which may be overcome by a body moving with the amazing velocity of light?'

At the fame time that M. Bofcovich fhews that many of the phenomena refpecting this laft-mentioned fubtile fubftance are much more easily folved upon this hypothefis than any other; he proves that it is by no means inconfiftent with any thing that we know concerning the laws of mechanics, or our difcoveries in natural philofophy.' Still further to illuftrate and to compleat our sketch of this curious fyftem, we fhall 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 treatife on the Immatetiality of the Soul. He there found, fays Dr. Priestley, that this Author's idea of matter was, that it confifted, as it were, of bricks, cemented together by an immaterial mortar. Thefe Ꮓ

REV. Oct. 1772.

bricks,

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