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formed, should suffer the same rise of temperature from a given quantity of caloric that the solid does. We learn, however, that it does not; it requires more caloric in the proportion, according to Dr Irvine, in the case of ice and water, of 10 to 8. Its сараcity has therefore been enlarged ; and with this fact it is impossible to maintain the theory of Dr Black, at least in its original form. The enlargement of capacity must occasion an absorption of caloric without any increase of temperature; or the cause, proved by experiment to exist, must necessarily produce the phenomena of latent heat; and the hypothesis of Dr Black is not only unnecessarily introduced, but seems to be awkwardly combined with a principle, of the truth of which we have experimental proof.
The simplicity of Dr Irvine's theory is its chief recommendation, and is singularly contrasted with the complicated hypothesis of Free and Combined Caloric. It regards this agent as existing in bodies only in one state ; that which produces the general effect, we denominate temperature. It is acknowledged as a law, unequivocally established, that different bodies require different quantities of it to produce the same temperature; and at any point, therefore, in the scale of the thermometer, they will contain unequal quantities. If one body be fixed on as a standard, another may contain more caloric at a given temperature, and this excess may be said to be latent, as not being discoverable by the thermometer. But there is evidently no propriety in the distinction, nor any reason to regard the excess of caloric in those which contain more than the assumed standard, as in any respect different from the other portion they contain, or from that which is contained in others. If we extend the same principle to the different states in which a body exists, as well as to different bodies, we adopt the most simple and comprehensive view which can be taken of the distribution of caloric; and if the principle be established by actual experiment, why should there be any hesitation in receiving it in this case as well as in the other? The two are perfectly alike. At a given temperature, water contains more caloric than ice, and it also contains more caloric than lead. The quantity which it contains above lead, is said to be owing to its greater capacity: why should not the same cause be assigned for the greater quantity it contains compared with that existing in ice? Even as an hypothesis, analogy would be directly in favour of such a conclusion and if established by experiment, the theory, in each of these cases, rests precisely on the same grounds.
be urged, perhaps, that, granting an augmentation of capacity to attend liquefaction or vaporisation, it is not proved to
be proportional to the absorption of caloric, which likewise takes place. It is indeed scarcely possible to do so. But why limit the theory, by supposing, without any proof, that they are not proportional ? or why render it complicated, by the introduction of an unnecessary assumption? If it were proved, indeed, that they were not proportional ; if it were shewn that part of the caloric which disappears does actually enter into a peculiar combination or state of existence ; or if there were any reason to believe that the phenomena attending the change of form could not be fully accounted for on the fact of a change of capacity--then, the limitation would be proper, and the charm of simplicity must be sacrificed to truth. But none of these points are established, and therefore they cannot be assumed as grounds of objection to the theory.
It is not shewn, nor, indeed, until an unexceptionable method be discovered of ascertaining the absolute heat of bodi other principle than the changes of capacity which they suffer in changing their form, can it be shewn, that the quantity absorbed is not proportional to the change of capacity. The difference between the capacity of water and that of ice, it may be said, is only as 10 to 9; yet in the conversion of ice into water, not less than 140 degrees of Fahrenheit are absorbed, while in the after elevation of the temperature of the water, the quantity requisite is only one tenth more than that necessary to raise equally the temperature of ice. But it is to be recollected, that the quantity absorbed at the moment of change is that which is necessary to preserve the temperature of the body in its new form, through the whole range of the thermometrical scale, from the point of absolute privation; and as we do not know with precision the extent of this range, it is an absolute impossibility to shew that the quantity absorbed is more or less than it ought to be; nor is there any reason, from any knowledge we have of the scale of temperature, to infer that it is,
Neither is there any sound reason for believing that the portion of caloric which disappears in liquefaction or vaporisation, enters into any combination more intimate, or at all different from that in which the rest of the caloric in the body exists. The doctrine of combined caloric has little solid support; but whether it be established or not, it has been improperly stated as opposed to the doctrine of Irvine on latent heat. It is true that Dr Irvine, and also Dr Crawford, maintained the position, that no portion of caloric exists in bodies in a combined state different from that which produces temperature, and that the whole they contain is proportioned to their capacities. But even this is not necessary to support
the theory, that the absorption of caloric, when bodies change their form, is owing to change of capacity. It might be admitted that a portion of caloric does exist in bodies in a combined state, without the conclusion following, that the quantity absorbed in liquefaction and vaporisation enters into such a state. The theory of this is to be inferred from the phenomena connected with it; and these give no countenance to the supposition that the caloric which disappears, or any part of it, enters into any pecuJiar or intimate combination.
Lastly, all the phenomena of latent heat are satisfactorily accounted for on the principle of a change of capacity. It is true, that it has been urged against this theory, that there are phenomena connected with the change of form for which it does not account. The great objection stated by Dr Black himself, and the only one not founded on misconception that has ever been urged against it is, that it does not account for the change itself. When a body is heated up to its melting point, what causes it to become liquid ? Not sensible caloric, it is said; for the liquid, at its formation, is of no higher temperature than the solid from which it has been formed. The change, therefore, it has been inferred, must be owing to part at least of the caloric which disappears, entering into combination with the solid; and on this latent heat the fluidity is supposed to depend, while it is not disputed but that another portion may become latent from a change of capacity.
If Dr Irvine junior has failed in any part of the defence of his father's doctrine,' we think it is in what relates to this objection. He first endeavours to obviate its force, by maintaining, in oppoposition to those who have hitherto adopted that doctrine, that it is not a just statement of it, that the form of a body is first changed, then its capacity enlarged, and lastly, that an absorption of caloric takes place.
• 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 fame 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 confequence of certain attractions or properties, which the ice and caloric are respectively possessed of.' p. 62.
Now, we are convinced that this view is erroneous, We have not been able to discover whether Dr Irvine himself has left any me-, morial of his opinion upon this subject; but we conceive it to be a necessary part of his system, that the form of a body is first changed; that by this the capacity is enlarged ; and that from this, again, a quantity of caloric is absorbed, and becomés latent. What other cause is to alter the capacity? Not surely the specific combination of a portion of caloric ; for this would be a modification of Dr Black's theory; and the question would recur, what determines this combination ? The cause must be assumed to be that change in the constitution of the body-in the arrangement of its particles, which accompanies fluidity. And this change of capacity, when it does happen, muft necessarily be immediately attended with an absorption of caloric, and must therefore be regarded as its cause. The three events are no doubt, to our observation, fimultaneous : but they are not actually so : the one must, momentarily at least, precede the other; or, on the opposite supposition, we shall have no distinct view of the difference in the theories of Irvine and Black.
Dr Irvine does not appear, indeed, to be very well satisfied with this view of his father's doctrine ; and he endeavours to shew that the objection may be otherwise obviated. Various powers, he conceives, may operate, the operation of which we cannot eafily trace; such as electricity, magnetism, and galvanism: and something ought to be allowed for such circumstances, when we reafon concerning the action of particles among each other.' p. 65. This reasoning the opponents of the system he defends will not, we are afraid, regard as satisfactory, and we conceive it to be altogether unnecessary. Did our limits admit of the discufsion, we believe that it would not be difficult to shew that the change must be ascribed to the operation of fenfible caloric alone ; that it is the result of its accumulating expansive power counteracting the cohesive attraction; that this change muit happen at an indivisible point in the thermometrical scale ; and that there is no force in this objection to Dr Irvine's system.
It has sometimes been said, that the view which Dr Irvine gives of these phenomena is ultimately the same with that given by Dr Black. Dr Black himself was sensible that it was not, and that Dr Irvine's theory was opposed to his. The same general fact, indeed, is admitted in both, or rather it is this fact for which they profess to account. A quantity of caloric is absorbed during liquefaction and vaporifation, without producing increase of tem. perature; but the cause afligned for this is very different in the one theory from what it is in the other. Dr Black always regarded this az existing in the fluid or vapour in some peculiar state different from that portion of caloric which produces temperature, and which he denominated sensible heat. Dr Irvine considered it as not in the least different, as to its mode of existence, from the rest of the caloric which the body contains, or from that which VOL. VIII. NO. 15.'.
is contained in others. In consequence of the change of form, that body is capable of containing more caloric than it could do, while in its former state, at the same temperature, or requires more to produce a given temperature ; and the caloric it abforbs, goes merely with the caloric it formerly contained, to preserve the temperature at which the change of form happened. Dr Black considered the change in the relation of the body to caloric as limited to the moment of liquefaction. Dr Irvine conceived the idea that it was not thus limited, but that this new relation was henceforward poffefsed by the fluid, and that in all subsequent elevations of temperature, the body, in this form, absorbs more caloric than it did while in the solid state. The two theories, therefore, whatever may be their merits, are totally distinct.
We have been told, too, that Dr Black's theory is simply the expression of the fact ; that it is a plain doctrine, which, to be disputed, must be misunderstood. It would be fo, if it merely announced, that when a solid becomes liquid, or a liquid is converted into vapour, a quantity of caloric is absorbed, which does not produce augmentation of temperature. But when it is affirmed that this portion of heat is the cause of fluidity, and is united with the body in some peculiar mode, to which its latent state is owing, it becomes an hypothesis, in opposition to which the theory of Irvine may be fairly opposed. Of their comparative merits, we believe few will doubt, who examine them with attention. Dr Black's may appear conformable to some loose analogies drawn from chemical combination. The other, independent of its experimental proof, is more simple and more conformable to the laws which caloric, in its relations to bodies, obferves. To Dr Black will remain the honour of having discovered and established, by a series of admirable experiments, the gerferal fact, that when bodies become liquid or aeriform, they abforb portions of caloric which do not augment their temperature; and to Dr Irvine, if we mistake not, will belong the praise of having given the just theoretical view of this important phenomenon. We consider the science as indebted to the editor for having brought the subject fully before the attention of chemists ; and we have little doubt that he will have the satisfaction of have ing contributed to establish his father's fame.
Another interesting subject connected with Dr Irvine's theory, it is known, engaged the attention of that philosopher--the determination of the real zero or point at which the scale of temperature commences, or at which bodies would be deprived of caToric. At what distance from a given temperature, Tuppose that of freezing water, will this be found ? It will be obvious to those who know any thing of the doctrines of heat, that such an