the virtues of a body depend. In the fame manner, the fubftance of gravity, in its departure, was accompanied by its effence, and left behind no weight,-but that of the falts and afhes.'

This however, he tells us (p. 163.), is not intended for a proof that gravity arifes from a diftinct species of matter; that fubje& he propofes to treat in a future volume; and then, we think, he fhould take into confideration the cafe of a coal fire made in a grate at some distance from the ground; and if it is found, notwithstanding the wafte of weight made by the fire, that there is no perceptible action or addition to the weight of a body held in a fcale in the space between the grate and the ground; he should help his readers to folve the difficulty thence arifing, how weight can defcend downwards without being perceived in the immediate space through which it paffes: otherwife they will probably fuppofe, that as the fubftance flies away in light, heat, odour, flame, and smoke, fo does the weight likewife, and that it is infeparable from body. For that bodies are feparated into particles, however fmall, and projected upwards by fire and air with great velocity, is furely no reafon for concluding that they are by that means deprived of their weight any more than is the ball of a cannon when fhot perpendicularly upward.

The fifth lecture contains the hiftory of fire; the elaftic matter identified with that fubftance, with Newton's ether, and with the electric matter. There is fomething curious, at least, and entertaining in this; but we cannot abridge it.

In the fixth lecture, the texture, compofition, and effence of the elastic matter are examined and defined. He fays, it is a continuous fubftance; i e. not compofed of atoms, or particles, that were orignally diftinct and feparated from each other; but one continued mafs, without pores or interftices, as any one fingle atom or particle is fuppofed to be in the modern phyfics, in which particles are faid to be impenetrable, poreless, and even indivifible. Yet he allows this continuous matter, though porelefs, to be penetrable, and even indefinitely divifible, farther than the mind can carry its conceptions of divifibility; which would be impoffible, he fays, were the component particles impenetrable and indivifible. The well-founded definition, as he calls it, of the elastic matter, is, that it is a fubftance compofed of two elementary principles, the expanfive, arifing from fire, and the coercive, from falt: fo blended and contextured together, as to form one homogeneous and continuous fubftance. Its effence confifts in a double power of expanfion and convergence; which it derives from the diftinct effences of its two components; and which may be expreffed by the name bridled expansion (a term borrowed from the great Sir Francis Bacon). One of its principles is the material caufe of cohefion, the other the fource of expanfion and fluidity; and the exertions of both, REV. Sept. 1786.

when

when excited to action, produce the phænomena of elafticity and tremor. This compound fubftance is the bafis of all lively material powers and qualities. Its activity is invigorated by heat, restrained by cold, but reduced and enfeebled by moisture.'

The fubjects of the 7th and laft lecture are, the propagation of light; its tranfmiffion through diaphanous bodies: the communication of heat, and the solution of transparency.

He will neither admit that light confifts of diftinct particles, nor that it is tranfmitted through the pores of diaphanous bodies. By way of illuftration, he propofes this queftion: If the pervading force of a leaden bullet, whofe diameter is an inch, be fuch as will make it pass through our air 200 yards in a fecond of time, what space in that medium shall a particle of light pas through, whofe diameter is but the thousandth part of an inch, and whofe velocity is only a million times greater than that of the bullet? He determines it to be the five millionth part of a yard in a fecond, according to his own way of working; and then afks, where now is the supposed tranfition of light through our atmosphere?

Now, though we allow that the particle, in his example, would be refifted by a number expreffed by unity and eighteen cyphers annexed of times more than the bullet, yet this, of itfelf, is no proof that it cannot be tranfmitted; for the force of refiftance is in the duplicate ratio of the velocity directly, and the given diameter inverfely; and therefore, if it once had any velocity in the medium, it must of neceffity be tranfmitted through it. Moreover, though we allow it to be true, that the density of the medium would be the greatest impediment of all, to fuch a particle paffing through it; yet this is of no force against the truth of the Newtonian doctrine, that the atmosphere and all diaphanous bodies are permeable by light; for it is expressly mentioned, with very strong reasons for its truth, that this is in fome measure owing to the attraction of the particles of the diaphanous body itfelf. Now, fuppofing the refiftance of the nedium ever fo great, yet ftill, if the attraction be but equal to that refiitance, which we think a reasonable fuppofition, no lofs of motion at all can enfue from refiftance, but the particles will be freely tranfmitted through the diaphanous medium with the uniform velocity that they poff ffed at their first entrance. Nor Can we conceive what Mr. O'Gallagher means by light propagated in orbem from centre to furface (p. 331.), flowing in continuum, without interval or interfice, with immenfe celerity, unremitted propagation, and rectilineal direction of the illumina tion (p. 333), neither communicated through transparent bodies by tranfition, nor compofed of diftin&t particles, but a flowing body, whofe parts are in fucceffive generation and confumption.'... Shall we ask if the air be a transparent body dif

tinct from light? Does not light pafs through it? We never faw any light but by means of air. What is light? if it has no tranfition through air, it is fomething of which we cannot poffibly have any notion. And if it flows through air, why not through glafs? is not what we fee by the fame within doors as without! But, fays Mr. O'G. (p. 380.), there is not any pervafion at all of the matter of light through diaphanous bodies; but that the light which they exhibit, and which occafions the phænomena of tranfparency, muft be that contextured throughout their conftitution in the form of elaftic matter.' So then we fhall presently be forced back to the blind man's queftion, and conclude with the poet,

For what light is, 'tis only light that fhews. Glafs is penetrable by fome one or more of the fine elaftic fluids, of which the air confifts; as is evident, from the well-known experiment, that if hot water be poured into it fuddenly, it will break; unless previously warmed by degrees. Nay, it is afferted as a known fact, that it is permeable by phlogifton. Then, why not by light? Or, how can it be a continuous fubftance void of pores, as this gentleman pretends to have demonftrated? (A fecond volume of Mr. O'Gallagher's work is published; but we have not yet feen it.)

Wi.

ART. VI. Essays on the Intellectual Powers of Man. By Thomas Reid, D. D. F. R. S. E. Profeflor of Moral Philofophy in the University of Glafgow. 4to. l. 5s. Boards. Edinburgh, Bell; London, Robinson. 1785.

TH

HE ingenious Author of the volume before us, published, about twenty years ago, a treatife entitled, "An Inquiry into the Human Mind, on the Principles of Common Senfe," of which we gave a favourable account in the Reviews for May and July 1764. The great object of the Author, in that work, was to refute certain principles with regard to the human underftanding, which had for a long time been commonly adopted by philofophers, and upon which Bishop Berkeley and Mr. Hume had at laft erected a fyftem of fcepticism. He endeavoured to afcertain those foundations upon which the truth and reality of human knowledge muft reft: and to fhew that the fciences which respect mind, as well as those which relate to body, ought to depend on first principles, which being felf-evident, admit of no proof, but cannot be denied without manifeft abfurdity. The inveftigation was confined to the five external fenfes, and as it was an avowed attack upon the philosophy of human nature that had been long in vogue, it presupposed, in the reader, fome acquaintance with the opinions and doctrines of Defcartes, Male branche, Locke, Berkeley, and Hume. The Author, notwithftanding his agreeable manner of writing, could not reafonably

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expect

expect that his work would be very univerfally read; yet it has been received with a degree of favour feldom fhewn to metaphyfical difquifitions; and it has undergone four impreffions.

The prefent treatife is written upon a much more extenfive plan than the former, and comprehends an account of all the powers of the human understanding. It alfo contains thofe things that may be confidered as elementary, in the feveral fubjects which are treated, as well as the conclufions deducible from them, and does not neceffarily require the reader's acquaintance with any former writer on the intellectual powers of man. The Author, through, the whole, difcovers a knowledge of the operations of the human mind, that must have been the refult of long and affiduous ftudy and reflection. The doctrines are ftated with a degree of accuracy and perfpicuity that is feldom to be met with in works of this nature; and em❤ bellished by illuftrations well chofen, and frequently entertaining. The opinions of others are distinctly and fairly stated, and when they differ from those which are embraced by himself, his objections are prefented with much, acuteness and penetration ; but at the fame time with temper and candour, and often with pleafantry and good humour. He has been equally fuccefsful in throwing a clear light upon the feveral branches of his fubject, and in diffipating those clouds in which they had been involved by the groundless theories and conjectures of fome ingenious men. By eftablishing the knowledge of mind upon fixed and felf-evident principles, fimilar to thofe, to which natural philofophy owes its prefent amazing progrefs, he has done a great and important fervice to fcience, which will probably be attended with valuable effects; and he has pointed out the proper means of oppofing the attempts of fcepticism. The operations of the human understanding were never more diftinctly explained than they are in this treatife; and no further recommendation is neceffary to thofe who think that "the proper ftudy of mankind, is man.'

Though the Author has modeftly given to this treatife the name of Effays, it does not, in fact, like thofe publications which often appear under the fame title, confift of detached pieces; but prefents the reader with a connected view of the feveral powers of the human understanding. The Effays are eight in number, and they might, perhaps with more propriety, have been denominated eight books.

The intellectual powers are commonly divided into fimple apprehenfion, judgment, and reafoning. By fimple apprehenfion we acquire thofe notions or ideas which are the materials of all our knowledge. By judgment we perceive the agreement or difagreement of our ideas, and accordingly affirm or deny fomething concerning them. By reafoning we deduce conclufions from two or more judgments. This divifion corresponds with

the

the manner in which the mind is ufually thought to proceed in acquiring knowledge. It firft receives ideas by means of the fenfes; thefe it compares, and forms judgments concerning them; and from its judgments, compared with one another, it deduces conclufions. Dr. Reid thinks (we believe with reafon) that there are fome operations of the underftanding that cannot be properly reduced to any of these three divifions, and, therefore, he follows a different enumeration.

Let us,' fays he, confider fome of the most familiar operations of our minds, and fee to which of the three they belong. I begin with confcioufnefs. I know that I think, and this of all knowledge is the most certain. Is that operation of my mind, which gives me this certain knowledge, to be called fimple apprehenfion? No, furely, Simple apprehenfion neither affirms nor denies. It will not be faid that it is by reafoning that I know that I think. It remains, therefore, that it must be by judgment, that is, according to the account given of judgment, by comparing two ideas, and perceiving the agreement between them. But what are the ideas compared? They must be the idea of myself, and the idea of thought, for they are the terms of the propofition, I think. According to this account then, first, I have the idea of myfelf, and the idea of thought; then by comparing these two ideas, I perceive that I think.

Let any man who is capable of reflection judge for himself, whether it is by an operation of this kind that he comes to be convinced that he thinks. To me it appears evident, that the conviction I have that I think, is not got in this way; and therefore I conclude, either that confcioufnefs is not judgment, or that judgment is not rightly defined to be the perception of fome agreement or difagreement between two ideas.

The perception of an object by my fenfes, is another operation of the understanding. I would know whether it be fimple apprehenfion, or judgment, or reafoning. It is not fimple apprehenfion, becaufe I am perfuaded of the existence of the object as much as I could be by demonftration. It is not judgment, if by judgment be meant the comparing ideas, and perceiving their agreements or difagreements. It is not reafoning, because those who cannot reaíon can perceive.

I find the fame difficulty in claffing memory under any of the operations mentioned.

There is not a more fruitful fource of error in this branch of philofophy, than divifions of things which are taken to be complete when they are not really fo. To make a perfect divifion of any clafs of things, a man ought to have the whole under his view at once. But the greatest capacity very often is not fufficient for this. Something is left out which did not come under the philofopher's view when he made his divifion : and to fuit this to the divifion, it must be made what nature never made it. This has been fo common a fault of philofophers, that one who would avoid error ought to be fufpicious of divifions, though long received, and of great authority, especially when they are grounded on a theory that may be called in queftion. In a fubject imperfectly known, we ought not

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