^{page 48 note *} Illustrations of the Huttonian Theory, p. 375.

^{page 49 note *} A few copies of the Considerations on Culm are still to be found in the shop of G. Elliot, Edinburgh.

^{page 50 note *} The Philosophical Society was instituted about the year 1739. The first volume of Essays was published in 1754; the second in 1756; the third in 1771. From the year 1777 to 1782, the meetings of the Society were pretty regular, much owing to the zeal of Lord Kames. Mr Maclaurin may be regarded as the founder of this Society.

^{page 59 note *} Dr Black's paper on magnesia, which contained this discovery, was communicated to the Philosophical Society of Edinburgh in June 1755, and was published in the second volume of their Essays, in the year following. Dr Hutton had at this time only begun his geological researches. It was not, I imagine, till after the year 1760 that they came to take the form of a theory.

^{page 59 note †} In the view here presented of the principle of compression, as employed in the Huttonian Theory, it is considered as a hypothesis, conformable to analogy, assumed for the purpose of explaining certain phenomena in the natural history of the earth. It rests, therefore, as to its evidence, partly on its conformity to analogy, and partly on the explanation which it affords of the phenomena alluded to. In supposing that it derives probability from the last-mentioned source, we are far from assuming any thing unprecedented in sound philosophy. A principle is often admitted in physics, merely because it explains a great number at appearances; and the theory of Gravitation itself rests on no other foundation.

The degree of this evidence will perhaps be differently appreciated, according to a man's habits of thinking, or the class of studies in which he has been chiefly engaged. To Dr Hutton himself it appeared very strong; for he considered the fact of the liquefaction of mineral substances by heat as so completely established, that it affords a full proof of the fusibility of those substances having been increased by the compression which they endured in the bowels of the earth. In his view of the matter, no other proof seemed necessary, and he did not appear to think that the direct testimony of experiment, could it have been obtained, would have added much to the credibility of this part of his system.

For my part, I will acknowledge, that the matter appears to me in a light somewhat different, and that though the arguments just mentioned are sufficient to produce a very strong conviction, it is a conviction that would be strengthened by an agreement with the results even of such experiments as it is within our reach to make. It seems to me, that it is with this principle in geology, much as it is with the parallax of the earth's orbit in astronomy; the discovery of which, though not necessary to prove the truth of the Copernican System, would be a most pleasing and beautiful addition to the evidence by which it is supported. So, in the Huttonian geology, though the effects ascribed to compression, are fairly deducible from the phenomena of the mineral kingdom itself, compared with certain analogies which science has established, yet the testimony of direct experiment would make the evidence complete, and would leave nothing that incredulity itself could possibly desiderate.

^{page 62 note *} To speak strictly, the law which connects the increments of humidity in the air with the increments of temperature, is not confined to any one of the three suppositions here made, but may involve them all. The humidity dissolved may be proportional to some function of the heat, that varies in same places faster, and in others flower, than in the simple ratio of the heat itself. Nevertheless, for that extent to which observation reaches, the reasoning of Dr Hutton is quite sufficient to prove that it varies faster; or, in other words, that is a curve be supposed, of which the abscissæ represent the temperature, and the ordinates the humidity, this curve, though it may in the course of its indefinite extent be in some places concave and in others convex toward the axis, is wholly convex in all that part with which our observations are concerned.

^{page 63 note *} It has been supposed that the chemical solution of humidity in air is necessarily implied in this theory of rain. The truth is, that the air is here considered only as the vehicle of the vapour, and that the transparent state of the latter is supposed to depend on the temperature, or the quantity of heat; but whether that heat act on the vapour solely and directly, or indirectly, by increasing the power of the air to retain it in solution, is, with respect to this theory, altogether immaterial.

Dr Hutton has indeed used the common language concerning the solution of humidity in air; but the supposition of such solution is not essential to his theory. He seemed, indeed, to entertain doubts about the reality of that operation, founded on the circumstance of evaporation taking place in vacuo. Experiments made by M. Dalton since the death of Dr Hutton, shew that there is great reason for supposing that the air has no chemical action whatever on the aqueous vapour contained in it. Manchester Mentoirs, vol. v. p. 538.

^{page 66 note *} It may be proper to mention here some useful observations in meteorology which Dr Hutton made, but of which he has given no account in any of his publications.

He was, I believe, the first who thought of ascertaining the medium temperature of any climate by the temperature of the springs. With this view he made a great number of observations in different parts of Great Britain, and sound, by a singular enough coinfidence between two arbitrary measures, quite independent of one another, that the temperature of springs, along the east coast of this island, varies nearly at the rate of a degree of Fahrenheit's thermometer for a degree of latitude. This rate of change, though it cannot be general over the whole earth, is probably not far from the truth for all the northern part of the temperate zone.

For estimating the effect which height above the level of the sea has in diminishing the temperature, he also made a series of observations at a very early period. By these observations he sound that the difference between the state of the thermometer in two places of a given difference of level, and not very distant, in a horizontal direction, is a constant quantity, or one which remains at all seasons nearly the same, and is about I° for 230 feet of perpendicular height.

I must, however, observe, that on verifying these observations, I have found the rate of the decrease of temperature a little flower than this, and very nearly a degree for 250 feet. This seems to hold for a considerable height above the earth's surface, and will be found to come pretty near the truth, to the height of five or six thousand feet. It is not however probable that the diminution of the temperature is exactly proportional to the increase of elevation; and it would seem that at heights greater than the preceding, the deviation becomes sensible; the differences of heat varying in a less ratio than the differences of elevation.

In explaining this diminution of temperature as we ascend in the atmosphere, Dr Hutton was much more fortunate than any other of the philosophers who have considered the same subject. It is well known that the condensation of air converts part of the latent into sensible heat, and that the rarefaction of air converts part of the sensible into latent heat. This is evident from the experiment of the air-gun, and from many others. If, therefore, we suppose a given quantity of air to be suddenly transported from the surface to any height above it, the air will expand on account of the diminution of pressure, and a part of its heat becoming latent, it will become colder than before. Thus also, when a quantity of heat ascends by any means whatever, from one stratum of air to a superior stratum, a part of it becomes latent, so that an equilibrium of heat can never be establislied among the strata; but those which are less, must always remain colder than those that are more, compressed. This was Dr Hutton's explanation, and it contains no hypothetical principle whatsoever.

To one who considers meteorology with attention, the want of an accurate hygrometer can never fail to be a subject of regret. The way of supplying this deficiency which Dr Hutton practised was by moistening the ball of a thermometer, and observing the degree of cold produced by the evaporation of the moisture. The degree of cold, cateris paribus, will be proportional to the dryness of the air, and affords, of course, a measure of that dryness. The same contrivance, but without any communication whatsoever, occurred afterwards to Mr Leslie, and being pursued through a series of very accurate and curious experiments, has produced an instrument which promises to answer all the purposes of photometry, as well as hygrometry, and so to make a very important addition to our physical apparatus.

^{page 69 note *} I Must take this opportunity of correcting a mistake which I have made in describing the junction in Glentilt, (Illustrations of the Huttonian theory, p. 310.) where I have said, that the great body of granite from which these veins proceed, is not immediately visible. This, however, is not the fact, for the mountains on the north side of the glen are a mass of granite to which the veins can be directly traced. This I have been assured of by Mr Clerk. Dr Hutton has not described it distinctly; and hot having seen the union of the veins with the granite on the north side, when I visited the same spot, I concluded too hastily, that it had not yet been discovered.

^{page 73 note *} For a fuller deduction of the conclusions here referred to, see Theory of the Earth, Vol. I. p. 458.; also Illustrations of the Huttonian Theory, p. 213.

^{page 74 note *} At what time these last speculations began to share his attention with the former, I have not been able to discover, though I have reason to believe that before I became acquainted with him, which was about 1781, he had completed a manuscript treatise on each of them, the same nearly that he afterwards gave to the world. His speculations on general physics were of a date much earlier than this.

The Physical System, referred to here, forms the third part of a work, entitled, Dissertations on different Subjects in Natural Philosophy, in one vol. 4to, 1792.

^{page 77 note *} Dissertations, &c. p, 312. § 31.

^{page 78 note *} Though Boscovich's Theory was published long before Dr Hutton's, so early, indeed, as the year 1758, there is no reason to think that the latter was in any degree suggested by the former. Boscovich's theory was hardly known in this country till about the year 1770, and the first sketches of Dr Hutton's theory are of a much older date. Besides, the method of reasoning pursued by the authors is quite different; and their conclusions, though alike in some things, directly contrary in others, as in what regards gravity, inertia, &c. The Monads of Leibnitz might more reasonably be supposed to have pointed out to Dr Hutton the necessity of supposing the elements of body to be unextended, if the originality of his own conceptions, and the little regard he paid to authority in matters of theory, did not relieve us from the necessity of looking to others for the sources of his opinions.

The principal defect of his theory seems to me to consist in this, that it does not slate with precision the difference between the constitution of those powers which simply form matter, and those that form the more complex substance, body. In other words, it does not explain what must be added to matter to make it body. The answer seems to me to be, that the addition of a repelling power, in all directions, is sufficient for that purpose. Such a repulsion, if strong enough, would produce both impenetrability and inertia. The matter, again, that possessed only an attractive power, like gravity, or a repulsive power only in a certain direction, like light, would not be inert nor impenetrable. In this inserence, however, from his system, I am not sure is I should meet with the author's approbation.

^{page 79 note *} See Dissertations V. and VI. on Matter and Motion, in the work aboye quoted. The Chemical Dissertation on Phlogiston is in the same volume, p. 171.

^{page 83 note *} Investigation of the Principles of Knowledge, Vol. III. p. 588. &c.

^{page 84 note *} I have hardly found this work of Dr Hutton's quoted by any writer of eminence, except by Dr Par, in his Spital Sermon, a tract no less remarkable for learning and acuteness, than for the liberality and candour of the sentiments which it contains.

^{page 85 note *} This was not the first attack which bad been made on his theory, for M. De Luc, in a series of letters, inserted in the Monthly Review for 1790 and 1791, had combated several of the leading opinions contained in it. To these Dr Hutton made no other reply, than is to be met with occasionally in the enlarged edition of his Theory, published four years afterwards. If I do not mistake, however, he intended a more particular answer, and actually sent one to the editors of the same Review, who refused to insert it. This, indeed, I do not state with perfect confidence, as I speak only from recollecion, and would not, on that authority, bring a positive charge of partiality against men who exercise a profession in which impartiality is the first requisite. Supposing, however, the statement here given to be correct, an excuse is still left for the Reviewers; they may say, that in communicating original papers, as they do not act in their judicial capacity, they are not bound to dispense justice with their usual blindness and severity, but may be permitted to relax a little from die exercise of a virtue that is so often left to be its own reward.

^{page 86 note *} For a defence of Dr Hutton against the charges here alluded to, I must take the liberty of referring to the Illustrations of the Huttonian Theory, p. 119, and 125.

^{page 92 note *} He had studied with great care several subjects of which no mention is made above. One of these was the Formation, or, as we may rather call it, the Natural History of Language. A portion of his metaphysical work is dedicated to the Theory of Language, vol. I. p. 574, &c.; and vol. II. p. 624, &c. He read several very ingenious papers on the Written Language, in the Royal Society of Edinburgh, see Transactions of the Royal Society of Edinburgh, vol. II. Hist. p. 5. &c. The Chinese language, as an extreme case in the invention of writing, had greatly occupied his thoughts, and is the subject of several of his manuscripts.

^{page 95 note *} A portrait of Dr Hutton, by Raeburn, painted for the late John Davidson, Esq; of Stewartfield, one of his old and intimate friends, conveys a good idea of a physiognomy and character of face to which it was difficult to do complete justice.