damp situation. Mr. Leslie, aware of these facts, thus employs them after asserting that the whiteness of plants is only a symptom of disease, he says,

:

It may be produced by other causes, which can only introduce morbid affections. For example, the stalks of culinary vegetables are blanched by heaping them with earth; since in the effort to convert its trunk into a root, the plant suffers languor and topical debility.' p. 164.

Our readers, we suspect, must have anticipated our remarks on this passage. The effect, it is said, may be produced by other causes, but general observation contradicts the assertion. Let us then examine the instance adduced in proof-the stalks of culinary vegetables are blanched by heaping earth around them, and this other cause of their whiteness is stated to be, the plant suffering languor and topical debility, from the effort to convert its trunk into a root. Who does not at once perceive that the blanching, in this case, can be imputed to no other cause but the exclusion of light, and who does not see that this figurative proximate cause, this languor experienced by the plant, from the effort to change its trunk into a root, is not only unnecessary, but insufficient, for the explanation of the phenomenon.

The remarks on the assumed objection, that the fluid of heat never displays itself in a separate collected state; as well as the corresponding note, (xviii.) abound with the most interesting and ingenious observations on the diffusion of heat, and on the theories of Drs. Black, Irvine, Crawford, &c.

In the tenth chapter, which also contains some ingenious remarks on the absolute Zero, or the beginning of the scale of heat, are several observations on the attraction of the corpuscles of different substances for the matter of heat, and on their corresponding expansions; on the absolute elasticity of the igneous fluid combined with bodies; on its density, on the quantity of matter which it actually contains; with several remarks on the emission and velocity of light; all of which manifestly originate in a mind richly fraught with those treasures of knowledge, which are absolutely necessary for suceeding in pursuits of this nature. The bold and noble spirit of enquiry, by which the author of this, truly scientific work has been directed, is no where more conspicuous than in his observations in this chapter, on the secular and annual expense of the solar substance, the tenuity and elasticity of light, and the various circumstances which affect its radiation. However ingenious may be the remarks which have been noticed in the preceding chapter, still they cannot be admitted to have established the fundamental position, which affirms the identity of heat and light. We cannot refrain from delivering it as our opinion, founded on a careful and impartial investigation of the reasoning here adduced, that the identity of light and heat

is by no means proved. Mr. Leslie thinks differently, as will appear from the following extract, which is chosen also as a spirited sketch of the proposed theory, and as an example of the elevated style which Mr. L. employs, when not trammelled by the detail of experiments, or of mathematical calculations.

The entire correspondence between theory and observation affords the most convincing evidence of the justness of our principles. It is therefore the same subtile matter, that, according to its different modes of existence, constitutes either heat or light. Projected with rapid celerity, it forms light in the state of combination with bodies, it acts as heat. Under this latter modification, it is more immediately the object of the present inquiry.

The igneous fluid absorbed into a solid substance, is not immoveably fixed and incapable of circulation. Disturbed by any external cause, it again diffuses itself, and restores the equilibrium. The particles of heat contained within a body, being attracted equally on every side, are left freely to exert their own expansive powers. If accumulated in one part, the increased elasticity there will occasion a flow towards the other parts.

'But though, in the circulation of heat, the substance which contains it is absolutely passive, the internal motions of that fluid must experiencé prodigious impediment and detention. Without such obstruction, its diffusion would be to sense instantaneous, having almost the celerity of fight itself. Had this been the constitution of nature, it might amuse the fancy to contemplate for a moment its vast and tremendous consequences. An uniform and unvarying temperature would have pervaded the globe: no distinction of climate, no vicissitude of seasons, and no grateful alternation of day and night. The azure vault of heaven, perpetually serene and cloudless, would lose its animated charms. If snow and hail would be unknown, so likewise would the refreshing influence of rains and dews. The face of the earth would present one monotonous picture of sterility no verdure to relieve the eye, no vegetation, and no sustenance for animals. All the springs of life would be locked up. The beneficial effects, the very existence, of artificial heat, would for ever have been concealed; for, the instant it was generated, it would spread and ingulph itself in the general mass.' pp. 188, 189.

The twelfth chapter begins with the assertion, that "it was shown, that a hot or cold surface propagates its influence with astonishing celerity through the air, only by exciting some peculiar energy in that active medium." This, however, we cannot suffer to pass without remarking, that the assumption is too confidently made, and is not warranted by the experiments alJuded to. By those experiments it appears, that some substance or other is influenced by hot or cold surfaces, that the effects of this influence are rendered particularly manifest by the concentrating power of concave mirrors, and that the propagation of this influence may be interrupted by the interposition of a screen, formed even of a transparent substance, applied so near to the hot or cold substance, that it may soon participate with it in tem

perature:

perature: the influence, at the focal point, becoming then manifest as before. As long as these phenomena will admit of another explanation, not more improbable than that which has been offered by Mr. Leslie, so long must his theory demand farther support; and there does not, in fact, appear to be one of these experiments which will not, at least, as well agree with the opinion, that the surrounding light, falling on the hot or cold substance acquires a proportionate change of temperature, which becomes manifest in the focal point, where this light is concentrated. That light should be the body, which is thus reflected, is more easy to be conceived, than that the vehicle conducting the heat should be the ambient air; and surely less difficulty accompanies the opinion, that light, in passing through a transparent screen, or reflected from its surface, may thereby obtain a change of temperature, and instantaneously manifest its effects on a distant point; than that this effect should be referred to the vehicle of certain oscillations excited in the surrounding air, traversing with a velocity, only to be equalled by that of the transit of light.

But waving our objections, and admitting, as Mr. Leslie supposes, that this rapid propagation of the influence of heat is effected by some peculiar energy in the surrounding air; we proceed to examine the opinion he offers, respecting the mode in which this rapid transit of heat is effected. This he supposes to be produced by an aerial motion, diverging from the source of action, and which may consist, either in the flight of the same particles, or in the successive transfer of agitation, by a vibratory impulsion, which shoots along a chain of particles, or through the general mass of fluid. The continuous flow of heated matter, by rapidly projected streamlets, Mr. Leslie says, is incompatible with the laws of fluids, and directly refutable by experiment. He therefore finds himself compelled to embrace the only alternative left him, and to refer the diffusion of heat, through the atmosphere, to the vehicle of certain oscillations, or vibratory impressions, excited in that elastic and active medium.

The next object of Mr. Leslie's research is therefore to discover the nature, origin, and subsequent propagation of these aerial vibrations by which heat is diffused, and to this is the rest of the twelfth chapter devoted. The mode which Mr. Leslie has adopted for the accomplishment of this object is to offer some pertinent observations on the theory of undulations, particularly on the nature and affections of the undulatory motions which are excited in the body of elastic fluids. The phenomena of sounds are undoubtedly employed by him with considerable success in explaining the physical operation of percussion, in inducing the vibratory agitations of an elastic fluid. The laws

N4

which

which regulate the effects of impulsion, are also happily illus trated; and the circumstances which modify the celerity, or apparent direction of the vibrations, are carefully considered.

All this is executed, in the remaining part of this chapter, with the hand of a master; and yet we have been mortified, at not finding that demonstration, which, in the next chapter (the thirteenth), our author takes the credit of having laid before us.

The only approach to the demonstration, thus assumed, or rather the first application of the theory of waves to explain the diffusion of heat, occurs after the demonstration is supposed to have been given. This appearing, as far as it extends, to yield a correct notion of Mr. Leslie's theory, we shall here place it in part before our readers.

"When heat penetrates, by its own activity, through a solid or inert mass, it successively dilates the several portions of matter which it encounters in its march. In the production of such multiplied displacements, it consumes its expansive energy; and its progress, therefore, is extremely slow. But if those intestine motions are generated by some extraneous cause, the heat, then suffering no impediment to its flight, will passively follow the tide of expansion. And such is the character of atmospheric pulses. The particles of air in immediate proximity to a hot surface, becoming suddenly heated, acquire a corresponding expansion, that propagates itself in an extended chain of undulation; and the minute portion of heat which generated the initial wave, thenceforth accompanies its rapid diffusive sweep. After a momentary pause, a fresh portion of heat is again imparted to the contiguous medium, and the act is continually repeated at certain regular intervals. The mass of air, without sensibly changing its place, suffers only a slight fluctuation as it successively feels the partial swell; but the heat attached to this state of dilation is actually transported, and with the swiftness of sound. Nor is the motion of the aerial pulses in any measurable degree retarded by the adhesion of the matter of heat, which is of such extreme tenuity, that, if not detained and cramped by the inertia of other bodies, the smallest possible force is sufficient to impel it with a celerity yet much inferior to that of light.

The same principle will likewise explain the dispersion of cold. For the atmospheric particles that come in contact with a cold surface, must suffer a sudden contraction, which will shoot its vibratory influence through the general mass: and the cold wave thus excited will, in its spreading tremulous flight, still retain the same distinctive character. Each of the minute parcels of air, as they successively feel a contractile disposition, will suffer a corresponding depression of temperature, or will permit a certain part of their heat to escape. The heat so liberated, is again instantly absorbed by the portion of air next behind, which, having contracted, is now recovering its tone. Though the motion of the aërial pulses, therefore, is the same as in the former case, yet the direction of the subtile element of heat is exactly reversed. Heat is, with the rapidity of sound, conveyed from all quarters to the cold surface, as to a common

centre.

These

These internal waves, whether of the quality of hot or cold, must evidently have all the properties which belong to elastic pulsations. Their motion is not apparently deranged by any mechanical agitation of the atmosphere and it was found, that the blowing strongly with a pair of bellows across the direction of the undulatory current, between the canister and the reflector, did in no perceptible degree affect the action on the focal ball. Each wave, or hemispherical shell, through the whole of its expansive sweep, retains the same absolute excess or defect of heat. But the intensity of this difference, or the partial elevation or depression of temperature, diminishing, therefore, in proportion as they spread, must, as in the case of radiations, be inversely as the square of the distance from its source.. It is not equal, however, in all directions; at right angles to the exciting surface, the power is greatest, and regularly declines on either side as the cosine of obliquity. The shell of aërial pulsation, it was shown, is not uniformly condensed or dilated, but after the law now stated: and these theoretical conclusions were abundantly confirmed by experiment. Nor, will the force or character of the undulations be altered in any respect, by traversing air of a very different or irregular temperature. Each distinct portion of that medium, being successively affected with a disposition to expand or contract, will likewise, at the same moment, assume the appropriate excess or defect of heat. A wave, for instance, that is originally hot, will always be hotter than the mass of fluid through which it travels: in fact, it will only superadd, in its passage, a certain measure of dilatation or of heat; and whether it encounters hot or cold streams, it will preserve the same relative excess of temperature. pp. 241.-245.

(To be continued.)

Art. IV. A Treatise of (on) The Laws for the Relief and Settlement of the Poor. By Michael Nolan, of Lincoln's-Inn, Esq; Barrister at Law. 2 Vols. 8vo. 11. 4s.. In boards, Butterworth, 1805.

THE 'HE importance of that system of our laws," Mr. Nolan observes in his advertisement," which respects the civil oeconomy and comforts of the poor is so obvious, that it is hoped an attempt to offer some facilities to the persons concerned in the administration of them will be received with indulgence."

The author's professed object has been, not only to unfold the theory and doctrine of the law, but also to supply the want of personal experience, by pointing out the manner of applying that theory to practice. He differs both in outline and arrangement from Dr. Burn and Mr. Const, enhancing the value of his work, by incorporating into it such cases of importance, as have been decided since the publications of former writers on this subject.

Mr. Const's treatise had the advantage of being grafted upon a work of established reputation and authority, and the editions through which it has passed shew the favourable opinion entertained of it by the public. It does not follow, however, that

nothing