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the virtues of a body depend. In the same manner, the substance of gravity, in its departure, was accomp nied by its essence, and left behind no weight,---but that of the sales and ashes.'
This however, he tells us'p. 163.), is not intended for a proof that gravity arises from a distinct species of matter ; that fustject he proposes to treat in a future volume; and.then, we chink, he should take into consideration the case of a coal fire made in a grate at some distance from the ground; and if it is found, not withstanding the waste of weight made by the fire, that there is no perceptible action or addition to the weight of a body beid in a scale in the fpace between the grate and the ground; he mould help his readers to solve the difficulty thence arising, how weight can descend downwards without being perceived in the imme. diate space through which it pasles : otherwise they will probably suppose, that as the fubstance Alies away in light, heat, odour, Aame, and smoke, so does the weight likewise, and that it is inseparable from body. For that bodies are separated into particles, however small, and projected upwards by fire and air with great velocity, is surely no reason for concluding that they are by that means deprived of their weight any more than is the ball of a cannon when shot perpendicularly upward.,
The fifth lecture contains the history of fire; the elaftic matter identified with that substance, with Newton's ether, and with the electric matter. There is sometbing curious, at least, and entertaining in this; but we cannot abridge it,
In the sixth lecture, the texture, composition, and essence of the elastic matter are examined and defined. He says, it is a continuous fubftance; i e, not composed of atoms, or particles, that were orignally distinct and separated from each other; but one continued mass, without pores or interstices, as any one single atom or particle is supposed to be in the madern physics, in which particles are said to be impenetrable, poreless, and even indivisible. Yet he allows this continuous matter, though poreJess, to be penetrable, and even indefinitely divifible, farther than the mind can carry its conceptions of divisibility; which would be impossible, he says, were che component particles impenetrable and indivisible. The well-founded definition, as he calls it, of the elastic matter, is, that it is a substance composed of two elementary principles, the expansive, arising from fire, and the coercive, from salt: so blended and contextured together, as to form one homogeneous and continuous substance. Its ellence confifts in a double power of expansion and convergence; which it derives from the distinct essences of its two compo. nents; and which may be expressed by the name bridled expansion (a term borrowed from the great Sir Francis Bacon). One of its principles is the material cause of cohesion, the other the source of expansion and Auidity; and the exertions of both, Rev. Sept. 3786. .
when excited to action, produce the phænomena of elasticity and tremor. This com pound substance is the basis of all lively material powers and qualities. Its activity is invigorated by heat, restrained by cold, but reduced and enfeebled by moisture.'
The subjects of the 7th and last lecture are, the propagation of light; its transmiffion through diaphanous bodies: the communication of heat, and the folurion of transparency. .
He will neither admit that light consists of diftinct particles, nor that it is transmitted through the pores of diaphanous bodies. By way of illustration, he proposes this question: If the pervading force of a leaden bullet, whose diameter is an inch, be such as will make it pass through our air 200 yards in a second of time, what space in that medium shall a particle of light país through, whose diameter is but the thousandth part of an inch, and whole 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 second, according to his own way of working; and then asks, where now is the supposed transition of light through our atmosphere?
Now, though we allow that the particle, in bis example, would be refifted by a number expressed by unity and eighteen cyphers annexed of times more than the bullet, yet this, of it. felt, is no proof that it cannot be transmitted; for the force of selistance is in the duplicate ratio of the velocity dire&tly, and the given diameter inversely; and therefore, if it once had any ve. locity in the medium, ic muit of neceffity be transmitted through it. Moreover, though we allow it to be true, that the density of the medium would be the greatest impediment of all, to such a particle pafling 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 Atrong reasons for its truth, that this is in fome measure owing to the attraction of the particles of the diaphanous body itself. Now, suppoling the refiftance of the inedium ever fo great, yet still, if the attraction be but equal to that rehilance, which we chink a reasonable supposition, no lofs of motion at all can ensue from refittance, but the particles will be freely transmitted through the diaphanous medium with the uniform velocity that they poflissed at their first entrance. Nor can we conceive what Mr. O'Gallagher means by light propagated in orbem from centre to furtace (p. 331.), Aowing in continuum, without interval or interfice, with immense celerity, unremitted propagation, and rectilineal direction of the illumina. tion (p. 333 ), • neither communicared chrough transparent bodias by tranfiiion, nor composed of diftinet particles, but a flowing nody, whole parts are in successive generation and conJumprioni'... Shall we ask if the air be a transparent body dir.
tinct tinet from light? Does not light pass through it? We never saw any light but by means of air. What is light? if it has no transition through air, it is something of which we cannot posfibly have any notion. And if it flows through air, why not through glass ? is not what we see by the same within doors as without? But, says Mr. O'G. (p. 380.), there is not any pervasion at all of the matter of light through diaphanous bodies ; but that the light which they exhibit, and which occasions the phænomena of transparency, must be that contextured through. out their constitution in the form of elastic matter.' So then we Thall presently be forced back to the blind man's question, and conclude with the poet,
For what light is, 'tis only light that shews. Glass is penetrable by some one or more of the fine elastic Huids, of which the air consists; as is evident, from the well-known experiment, that if hot water be poured into it suddenly, it will break; unless previously warmed by degrees, Nay, it is asserted as a known fact, that it is permeable by phlogiston. Then, why not by light? Or, how can it be a continuous substance void of pores, as this gentleman pretends to have demonstrated ?
(A second volume of Mr. O'Gallagher's work is publilhed; but we have not yet seen it.)
Wi. ART. VI. Elays on the Intelle&tual Powers of Man. By Thomas
Reid, D.D. F.R.S. E. Professor of Moral Philosophy in the University of Glasgow. 460. il, ss. Boards. Edinburgh, Bell; London, Robinson. 1785. THE ingenious Author of the volume before vs, published, T about twenty years ago, a treatise entitled, “ An Inquiry into the Human Mind, on the Principles of Common Sense,” 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 understanding, which had for a long time been commonly adopted by philosophers, and upon which Bishop Berkeley and Mr. Hume had at laft erected a system of scepticism. He endeavoured to ascertain those foundations upon which the truth and reality of human knowledge must reft: and to thew that the sciences which respect mind, as well as those which relate to body, ougbt to de. pend on first principles, which being self-evident, admit of no proof, but cannot be denied without manifeft absurdity. The investigation was confined to the five external senses, and as it was an avowed attack upon the philosophy of human nature that had been long in vogue, it presupposed, in the reader, some ac. quaintance with the opinions and doctrines of Descartes, Male, branche, Locke, Berkeley, and Hume. The Author, notwithftanding his agreeable manner of writing, could not reasonably
expect expe&t that his work would be very universally read; yet it has been received with a degree of favour seldom Thewn to metaphyfical disquisitions; and it has undergone four impressions.
The present treacise is written upon a much more extensive plan than the former, and comprehends an account of all the powers of the human understanding. It also contains those things that may be considered as elementary, in the several subjects which are treated, as well as the conclusions deducible from them, and does not necessarily require the reader's acquaintance with any former writer on the intellectual powers of man. The Author, through the whole, discovers a knowledge of the operations of the human mind, that must have been the result of long and affiduous study and reflection. The doctrines are stated with a degree of accuracy and perspicuity that is seldom to be met with in works of this nature, and embellished by illustrations well choren, 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 presented with much, acuteness and penetration ; but at the same time with temper and candour, and often with pleasantry and good humour. He has been equally successful in throwiog a clear light upon the several branches of his sube ject, and in diffipating those clouds in which they had been involved by the groundless theories and conjectures of some ingenious men. By esta lishing the knowledge of mind upon fixed and self-evident principles, fimilar to those, to which natural philosophy owes its present amazing progress, he has done a
great and important service to science, which will probably be . attended with valuable effects; and he has pointed out the proper means of opposing the attempts of scepcicism. The operalions of the human understanding were never more diftin&tly ex, plained than they are in this treatise; and no further recommendation is necessary to those who think that “the proper study of mankind, is man.”
Though the Author has modestly given to this treatise the name of Efrays, it does not, in fact, like those publications which often appear under the same title, consist of detached pieces; but presents the reader with a connected view of the several powers of the human undertanding. The Essays are eight in number, and they might, perhaps with more propriety, have been denominated right books.
The intellectual powers are commonly divided into simple apprehenfion, judgmeni, and reasoning. By simple apprehension we acquire those notions or ideas which are the materials of all our knowledge. By judgment we perceive the agreement or disagreement of our ideas, and accordingly affirm or deny fomething concerning them. By realoning we deduce conclusions from two or more judgmenis. This division corresponds with
the manner in which the mind is usually thought to proceed in acquiring knowledge. It first receives ideas by means of the senses; these it compares, and forms judgments concerning them; and from its judgments, compared with one another, ic deduces conclusions. Dr. Reid thinks (we believe with reason) that there are some operations of the understanding that cannot be properly reduced to any of these three divifions, and, therefore, he follows a different enumeration.
Let us,' says he, consider some of the most familiar operations of our minds, and see to which of the three they belong. I begin with consciousness. 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 simple apprehenfion? No, surely, Simple apprehension neither affirms nor denies. It will not be said that it is by reasoning that I know that I think. It remains, therefure, 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 proposition, I think. According to this account then, first, I have the idea of myself, 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 con. vinced 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 consciousness is not judgment, or that judgment is not rightly defined to be the perception of some agreement or disagreement between two ideas."
· The perception of an object by my senses, is another operation of the understanding. I would know whether it be simple apprehenfion, or judgment, or reasoning. It is not simple apprehension, be. cause I am persuaded of the existence of the object as much as I could be by demonstration. It is not judgment, if by judgment be meant the comparing ideas, and perceiving their agreements or disagreements. It is not reasoning, because those who cannot reason can perceive,
• I find the same difficulty in classing memory under any of the operations mentioned.
There is not a more fruitful source of error in this branch of philosophy, than divisions of things which are taken to be complete when they are not really so. To make a perfect division of any class of things, a man ought to have the whole under his view at once. But the greatest capacity very often is not sufficient for this. Something is left out which did not come under the philosopher's view when he made his divifion : and to suit this to the division, it must be made what nature never made it. This has been so com. mon a fault of philosophers, that one who would avoid error ought to be suspicious of divifions, though long received, and of great authority, especially when they are grounded on a theory that may be called in question. In a subject imperfectly known, we ought not