A Philosophical Essay concerning Light. By Bryan Higgins, M. D. Vol. I. 6s. Dodiley,

(Continued from Vol. V. Page 134*.) In this age of philosophical improvement, when the most interesting discoveries in physics, promise a farther inlight to first principles, than the most fanguine, experimentaliit hath hitherto dared to hope for, the pursuits, of every enquirer into the secrets of nature, become a peculiar obje&t of critical attention. We shall, for this reason, beltow a greater portion of that attention to the present work, than otherwise its bulk or fubje& might seem to r quire. In opening this article in a former Review, we gave a specimen of Dr Higgins's manner of argumentation, in laying down the definitions proper to his pian, in the introduction to his Essay.

As a trilling flip at the threshold, however, may be of material consequence in the prosecution of any intended career, it is highly necessary that it shoull be avoided or corrected.

It is with great propriety Dr. Higgins obterves, in fettling the minima naturæ, or primary elements of bodies, that inde

The indispensable avocations, of the writer of this article, having obliged him hitherto to defer its continuation ; as it has done that of fume other articles ; which shall shortly be compleated. Vol. VI. L


finite magnitude and infinite divisibility, however justly applied by mathematicians in treating of mere extension, ought to have no place in physical enquiries. Ideal divisions, therefore, of palpable bodies should certainly be rejected. But in rejecting ideal divisions should not we also reject ideal qualities?

I consider,” says our author, “the smallest parts into which any mass of matter is ever divided in the process of nature or art, as the ultimate parts of that mass, and as finall bodies which are incapable of actual division or diminution. These minute bodies are very aptły termed atoms; and using the term atom in this sense, I express by it no more nor less, than what really exists,” But what is this? Surely this is neither more nor less than a very equivocal mode of expression! Has an atom existence without elence? What is the property or quality of these indivisible atoms? - Are such atoms only finaller bodies? If so, what constitutes their priinitive folidity? It has been maintained, with great reason*, that the folidity of bodies is as mere a phænomenon as any other property attending them. If every condition, as our author terms it, of


so which is not deducible from experience, or necessary towards explaining natural phænomena, is to be rejected,” the supposed folidity, or impenetrability, of the primary elements, must be rejected; as being equally ideil with their infinite divisibility: Not that we mean to reduce such elements to mere inathematical points, or to dispute their existence; we only mean to enforce the necesity of ascertaining their essence. And this may be more fully and fairly deduced from an argument a priori, than from physical experiment -The actual division, and thence the evident divisibility, of palpablc bodies, into impalpabie parts, is a fufficient proof that the particles, mslecules, atoms, or whatever else our author chufes to call them, are so ininute as to elude the investigation of experiment, with respect to their magnitude, figure, or form. That they are folid or impenetrable, therefore, it is impossible for us to know by experience; and that such solidity is necessary to explain the phænomena of nature, cannot be granted, if such phænomena are to be more fimply and mechanically explained without it --Sir Isaac Newton, indeed, in his Regulæ philofophandi, lays down a rule, which inay keep Dr. H. in countenance, while he concludes that the component elements of bodies must posseis the same prop rty as the bodies themselves. “We find,” says he, " that leveral bodies are hard; and argue, that

* by Dr. Luzac of Leyden, and many other ingenious phyfiologists.


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the hardness of the whole only arises from the hardness of the parts: whence we infer, that the particles not only of perceprible bodies, but of all others, are hard likewise.” But is this a philosophical way of arguing?. Might not it be said, with equal propriety, We find several bodies are soft, and therefore their primary particles, as well as thote of all others, are foft too?" We will venture to affirm, with an able modern writer, that there never was, nor can be, an experiment made, from which we can fairly infer, that folid or impenetrable elements, or atoms, exist in nature.

That refiftinų objects exist, we feel, and know. But that their relistance is the effect of the absolute impenetrability of their conftituent parts, we neither can feel, nor know. The hardness of palpable objects (and such only can come under the scrutiny of physical experiment) is merely relative.

By experience we learn, that soine bodies are comparatively inore soft or hard, penetrable or iinpenetrable, compressible or incompreffible, than others; but, when we shall have found tlie hardest and most compact body in nature, we shall only have found a body tliat is impenetrable to others leis penetrable. We have no means to make proof of its own absolute solidity ; for, even the substance of soft clay is as impenetrable to foft ciay as that of ironi to iron. And the body which appears hard and impenetrable as iron to the gentle pressure of a loft hand, would appear toft and penetrable to the forcible impulse of a band as hard as iron,

When Sir Isaac Newton, therefore, inferred from the extenfion, hardness, and weight of palpable bodies, that the primary homogeneous elements of which all bodies are constituted, must be extended, folid, and heavy too, he reafoned il. logically ; as well as took that for granted which he should have proved.

We do not, however, as before observed, question the exiftence of such atoms; but what are they? To define a thing, by giving it a naine, denoting neither more nor lets than that it exists, is no definition at all. That such primitive atoms do extend, or describe a certain quantity of space, how fmall soever, is the necellary coniequence of their number, or plurality: even two mathematical points being necefíarily divided by some positive distance, which is divifible into at least three other points; as necefianly divided by as many others. Either then the priory aicins iuft leverally deicribe, or occupy, fomne certain quantity of trace themselves, or they must be divided by something elie that does. Now it is ctr


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tainly more philosophical to impute extension to the primary elements themselves than to the mere mediuin of their divifibility. Every such atoin muft of neceffity, therefore, occupy some portion of space, or be of some dimensions ; but whether these dimensions be fixed or changeable, whether such atoms may not be capable of dilatation and compresfion, are questions no physical experiment can immediately decide.--If we banish ideal conditions, and impute to things, barely found to exift, such qualities only as they cannot be without, we shall attribute to ruch atoms merely the property of expansion or capacity of occupying : he quantity of space, described by them, and nothing more This property, however, is, for the reasons before given, far from being that of solidity or impenetrability, It will be fimply that of a power of expansion; viz. that of resisting an equal expantive force, being in contact with, or acting in a contrary direction.-And fuch, we presume, are the primary elements, or atoms, of all material bodies. ---It is not our business to Thew here in what manner the folidity and other phænomena of bodies mechanically result, from the motion of such elements among each other. We have just hinted at the only property, we can philosophically align such elements, in order to thew how liberal our author is in his portue lata of first principles : even Sir Isaac Newton being still leis so than Dr. Higgins. Sir Isaac, indeed, presumed that the primary elements were of a determinate form, folid, and impenetrable ; but then he conceived them to be inert and inactive. Even their supposed innate power of attraction, he admitted, might be the effect of external impulse. Dr. H. on the contrary conceives the attraction of matter to be the innate and inseparable quality of the primary atoms.

“ As we are convinced by experience, that the whole weight of any body, is equalled by the sum of the feveral weights of the smalieit parts thereof which we can examine ; and that the gravitation of any large body confits of the gravitations of these paris; and as gravitation is never found to ceale, by reason of any further division of mafles; and as aerial and alkaline and other chillic fluids do gravitate when their ultimate parts are held distant from canh other, as well as when these parts cohere and form folid ponderous bodies; it is reatonably inferied that cach atom of an hevy homogene.I mats, doth gravita: e according to the fame laiv which regulates the gravitation of the inafs.

" in a variety of chemical operations we learn that the attractive powers, whereby malies of earth or of acid and alkali, or of any other kind of matter, are comlined or made to cohere, do operate 10 the faine ettect on the smallest vitible parts of thele malles : and wlien the parts of folid bodies become invisible in foluions, we find the uliniate invisible parts are actuated by attractive powers, which caule

them to unite and form visible particles or large maffes, in proportion as the menttruum is withdrawn: and in the mixture of invisible elas. tic fluids which condense each other, we perceive that these attractive powers actuate the ultimate parts of such Huids, as well as the visible parts of the inaffes which are formed of them : and thus we are authorized to conclude that each atom attracts according to the law which regulates the attraction of the bodi, consisting of any number of such atoms; and that the law of attraction which is dis overable in a body which we can examire, is the law of attraction of the homogencal atoms thereot, and of any one of these, which by reason of its minuteness we cannot examine.”

We are by no means fatisfied with this inethod of reasoning, by induction, as our author terms it: nor are we clear that gravitating bodies may not be resolved into parts that will not gravitate. On the contrary, we have many good reasons, in the like mode of induction, to conclude, they may. In the next section our author maintains, that “ the atoms of matter are immutable in figure, and may, without fenfible error, be considered as globular bodies !”— According to the system above-hinted, those particlcs are mutable in figure, although they must, if at rest with respect to each other, assume an hexagcnal one. The figure of bodies, depending on their property of folidity, we conceive to depend on the inotion.of their constituent parts, as do also their texture and density; differing from the notion of Sir Ilaac Newton only as imputing that to mechanical impulse which he imputes to physical attraction, a difference of no importance in the present cate, as our ingenious chemist controverts both the cause and effect, as laid down by that great philosopher. “ Bodies, says Dr. Higgins, are not mutable into each other, and the properties of the atoms of any elements are indefeasible.”—Sir Isaac Newton had said, in his famous Queries, printed at the end of his Optics,

“ Now the finallest particles of matter may cohere by the itrongest “ attractions, and compose bigger parricies of weaker virtue ; and “ many of these may cohere and compose bigger particles wh-re virtue “is itill weaker, and so on tor divers fucceflions, until the progresion "end in the biggest particies on which the operations in chemilliy, and " the colours of vatural bodies depeod, and which by cohering com

pole bodies of a fenfille magnitude, &c.".

to Much streis is laid on this and the subsequent passage ly those who imagine, that the smallest physical parts of matter ditfer from each ciher in no respect; that by ilie union of two or more of these, particles are formed pollelling different properties, according to the num. ler and arrangement of their puits; i la luch particles as are computed of the same number of paris, constituie ihe portions of manier which 7 full Elements; and that particies confulting of unequal numbers o. parts,


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