صور الصفحة
PDF
النشر الإلكتروني

It is to the well directed experiments of Dr. Black, that we are indebted for an answer to these questions. He mixed a pound of ice and a pound of water, each at 32°, in separate vessels, with a pound of water of a higher temperature. The mixture of the two pounds of water, he found, possessed a degree of temperature, which was nearly the mean of their former temperatures. But on examining the ice and water, he discovered that the temperature was lower than the mean, and that 140° of heat had disappeared: considering that this had actually entered into the water, during its liquefaction, although its presence was not manifested by a proportionate increase of temperature, he designated it by the term of latent heat. Pursuing this course of inquiry, he also found, that during the conversion of water into vapour a considerable quantity of heat disappeared, having been imbibed by the steam. Thus Dr. Black shewed that different bodies possessed powers of containing heat peculiar to each individual substance, and that these powers were independent of the comparative bulks or weights of these substances. The heat which thus disappeared, he believed, entered and existed in the substance, in a peculiar state, different from that in which it existed while capable of affecting the thermometer. He also supposed that the changes of form from solidity, to fluidity, and to vapour, were caused by the introduction of this latent heat. Thus was he led to determine the existence of a general law of nature-That all bodies passing from a solid to a fluid state, and from that to a state of vapour, imbibe a vast portion of heat, the presence of which is not indicated by any affection of the thermometer. The view which Dr. Black had taken of the subject did not, however, appear to Dr. Irvine sufficiently comprehensive. He rather thought it possible, as we are here informed, that the capacity for heat, possessed by water and other fluids, might be found to exceed that of ice and their relative solids; and that hence the sensible heat would diminish as the capacity of the body increased and pice versa. Thus a solid body, suppose at 20° becoming instantly fluid and absorbing 10° of heat, would then manifest only the 10 remaining degrees of heat, and would become 100 colder without any heat being taken out of it and on the other hand, the same becoming instantly solid, would become 10° warmer without any additional heat being thrown into it.

With the view of determining, whether any difference existed between the capacities of ice and water, he made the necessary experiments; and by these ascertained the capacity or relative heat of water to that of ice to be, in a ratio of 10 to 8; and, extending his experiments to other bodies, he was enabled to infer, that it was a general law of nature, that

the capacity of all bodies for heat is increased by fusion, and that of all fluids by vaporisation.

The habits of substances with respect to heat, it may be observed, also undergo a change, with the change of their form and capacity. By passing from a solid to a fluid state, a body' is changed from one which is easily heated, to one which is heated with difficulty, and which requires a greater quantity of caloric to raise it a certain number of degrees, agreeable to the discovery made by Dr. Black, of the great quantity of heat necessary to the conversion of a solid substance into a fluid.

The theories, both of Dr. Black and Dr. Irvine, agree in establishing the entrance of large quantities of caloric into bodies, during their fusion; but Dr. Irvine did not consider this enlargement of the specific heat of bodies in a fluid state as satisfactorily explained. He considered that the quantity of specific heat would always be in proportion to the capacity of any body for heat; and instead of considering the heat which disappears, in these cases, and which Dr. Black termed latent heat, as likely to have entered into any peculiar or unusual combination or form, he believed that the caloric existed there precisely in the same way, as at all other times, and was discoverable by an appropriate test.

The objections which Dr. Black alledged against this theory were, that admitting the doctrine of capacities to be well founded, still the inferences were not warranted, and that it did not account for the principal phænomenon, the change of the solid into a fluid. But Dr. Irvine jun., referring to the arguments which have been employed by Dr. Black and others against this theory, states that they derive a great portion of their force from a misconception and consequent mis statement of Dr. Irvine's theory; it having been assumed, he says, as a part of the theory, that the capacity of the ice is first enlarged, and then the quantity of caloric is admitted, which disappears. By this statement, Dr. Irvine contends, more points in the explanation are included, than are, by any supposition required. The theory, he is however convinced, may be defended, whether Dr. Black's account of it, or his own, be admitted. The recapitulation of Dr. Irvine's own account of the theory is thus given.

The solid differs from its relative fluid, when both are of the same temperature, in these circumstances, merely, that the capacity of the former is less than that of the latter, and that of consequence, the heating of both beginning at the natural zero, more caloric is necessary for the elevation of the temperature of the water, than for that of the ice. The difference between the whole heat in water at 32°, and the whole heat of ice at 32°, is called the latent heat of that body, and ice being converted into water,,

requires this quantity of caloric to retain its temperature at the same degree as before. But this caloric does not enter the ice before its capacity is changed. Much less is the capacity enlarged before the caloric enters the body. These events are synchronous, and are neither cause nor effect of each other, but are mutually the consequence of certain attractions or properties which the ice and caloric are respectively possessed of. How these substances have such attractions, we are far from pretending to explain. But it is conceived that this theory ought no more to be required to explain the cause of attraction, than other theories, on this, and various chemical subjects, none of which afford any explanation of such difficulties.' p. 62.

The reason for thus contending for the simultaneous performance of these processes, may perhaps be still farther to distinguish this theory from that of Dr. Black; but that is certainly unnecessary. Dr. Irvine's theory is, in every respect, sufficiently distinct from Dr. Black's, and possesses the merit of being simple and explicit, and of being in perfect agreement with all the phenomena; to load it with this synchronous operation, we cannot therefore consider to be just or politic The theory of Dr. Irvine is, in every other respect, explained and supported by his son, with a perspicuity, and a strength of argument, which afford the strongest proofs of his learning and abilities. Nor can we discover, that the zeal and enthusiasm which he must have felt, have ever been allowed to pervert his judgement or candour.

In the succeeding essay, "On the capacities of bodies for heat," are introduced some very ingenious observations on the capacities of different bodies. Some bodies, as bees-wax, spermaceti, &c., pass from the solid to a fluid state, through various degrees of softness, which at last terminate in perfect fusion. These, it was concluded both by Dr. Black and Dr. Irvine, take in a part of their latent heat during their softening, and give it out again during their gradual hardening: it may therefore be assumed, on the principles of Dr. Irvine's theory, that these substances change their capacities for heat gradually. Indeed, it seems just to agree with him in supposing, that there is scarcely an instance, in all the phenomena of nature regarding caloric, where heat is produced, or temperature raised, without a corresponding change of capacity. This coincidence of change of temperature, and of capacity, offers itself indeed so perpetually to our view, that to doubt of it would be to deny the evidence of our senses. But their relation, as cause and effect, is a subject, to which our investigations may still with propriety be directed.

It is indeed necessary to remark, that the view which Dr. Irvine has taken of this part of the subject, does not seem exactly to accord with the opinions which he has advanced

in the passage already quoted. We are there told that the capacity is not enlarged before the caloric enters the body. These events are synchronous, &c. But in discussing the question of cause and effect, the Doctor has thus expressed himself.

But, to what is this almost universal coincidence of the change of capacity and the change of temperature to be attributed? Ought we not to conclude, that the one of these must, in some way, be the cause of the other, and since the change of temperature cannot be made to account for the change of capacity, that the change of capacity which affords an adequate explanation of the alteration of temperature, is the cause of that alteration.' pp. 102.

Now, if the change of capacity be admitted to be the cause of the alteration of temperature, the synchronous occurrence of the phenomena can surely no longer be supported. The change of capacity, as a cause, must precede the admission or expulsion of caloric, which as a cause, must also precede the alteration of temperature. The successive occurrence of these circumstances appears to be indubitable.

The next essay is on the lowest degree of heat, or that point at which bodies are wholly deprived of caloric. In this essay, a mode is proposed of ascertaining the natural zero, founded upon the consideration of the change of the capacity of bodies, during their fusion; and of the quantity of calorie necessary to produce fluidity. The calculations employed for this purpose are much too long to allow of their being transferred to these pages; and to convey them in an abridged form is impossible; we must therefore refer the scientific chemist to the volume itself.

Of the fourteen essays on various subjects, by the late Dr. Irvine, we can only speak in general terms; from the quantity and variety of information watch they convey, they must prove highly gratifying to those who can derive pleasure from tracing those laws, by which the operations of material nature are regulated.

From these essays and the accompanying notes by Dr. Irvine, jun., we perceived with pleasure the considerable advances which the late Dr. Irvine had made, towards completing some of those discoveries, which have since obtained for others the highest philosophic honours. In his essay on the fertility of soils, filled with most useful and ingenious observations, an allusion is made to the power of plants to decompose fixed air, and to restore the air to the atmosphere in its original state. It appears, that Dr. Irvine always considered himself to have been the first who suggested the probability of this power of vegetation to resolve carbonic acid into its

principles, and thus restore the purity of common air. It also appears that as early as 1771, Dr. Irvine, in a paper on water publicly read at Glasgow, declared his opinion, that chalk was soluble in water, impregnated with carbonic acid; without possessing any knowledge of Bergman's experiments, on this subject, which were published to the world in 1774.

The work concludes with two Essays by Dr. Irvine, jun. In the first of these, on latent heat, the Doctor endeavours to compare its quantity, in various instances, and to discover any principle by which its entrance into fusing bodies may seem to be governed. The result of his inquiry, however, is not very satisfactory: the latent heat of a few substances appears to be ascertained; but no ratio is discovered, by which the quantity of caloric of fluidity is generally regulated.

The second of these essays, on the affections of sulphur with caloric, is chiefly composed of conjectures, on the property possessed by sulphur of becoming thicker by protracted exposure to heat; a subject on which we must be contented to wait for farther observations.

The scientific world is much indebted to Dr. Irvine jun., for the publication of this volume, the variety of subjects to which it refers, and the ingenuity and judgement with which they are discussed, will doubtless secure that favourable reception, which, from the pleasure we have felt in the perusal, we sincerely wish it may obtain.

Art. VII. Short Discourses to be read in Families; by W. Jay. 2 vols. 8vo. pp. 850. price 16s. Williams and Co. Hatchard. 1805.

A Late traveller in France observes, that preachers in that

country dare not push themselves into notice by publishing their sermons, unless their reputation has been previously well established. Were an individual so far to yield to his own vanity, or the applauses of his particular audience, as to set this custom at defiance, he would expose himself both to public ridicule, and to the censures of his diocesan for his unmortified pride. But we now review a preacher, whose celebrity gives him a right to publish, and places him beyond the necessity, or perhaps the wish of claiming our forbearance. We shall, therefore, feel at our ease, to condemn faults which many may be tempted to imitate, or applaud merits which others have already admired. His reputation, indeed, is not confined to this country; for we perceive by some recent periodical works from America, that his works are received with much approbation by the scrupulous theologians of the New World.

« السابقةمتابعة »