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It is unnecessary to carry our conjectures concerning the train of Dr Hutton's discoveries to a greater length; the developement of the principles now enumerated, and the comparison of the results with the facts observed in the natural history of minerals, led to those discoveries, by a road that will be easily traced by those who study his theory with attention.

It might have been expected, when a work of so much originality as this Theory of the Earth was given to the world, a theory which professed to be the result of such an ample and accurate induction, and which opened up so many views, interesting not to mineralogy alone, but to philosophy in general, that it would have produced a sudden and visi

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.

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ble effect, and that men of science would have been every where eager to decide concerning its real value. Yet the truth is, that it drew their attention very slowly, so that several years elapsed before any one showed himself publicly concerned about it, either as an enemy or a friend.

Several causes probably contributed to produce this indifference. The world was tired out with unsuccessful attempts to form geological theories, by men often but ill informed of the phenomena which they proposed to explain, and who proceeded also on the supposition that they could give an account of the origin of things, or the first establishment of that system which is now the order of naMen who guided their inquiries by a principle so inconsistent with the limits of the human faculties, could never bring their speculations to a satisfactory conclusion, and the world readily enough perceived their failure, without taking the trouble to inquire into the cause of it.

ture.

Truth, however, forces me to add, that other reasons certainly contributed not a little to prevent Dr Hutton's theory from making a due impression on the world. It was proposed too briefly, and with too little detail of facts, for a system which involved so much that was new, and opposite to the opinions generally received. The descriptions which it contains of the phenomena of geology, suppose in the reader too great a knowledge of the

things described. The reasoning is sometimes embarrassed by the care taken to render it strictly logical; and the transitions, from the author's peculiar notions of arrangement, are often unexpected and abrupt. These defects run more or less through all Dr Hutton's writings, and produce a degree of obscurity astonishing to those who knew him, and who heard him every day converse with no less clearness and precision, than animation and force. From whatever causes the want of perspicuity in his writings proceeded, perplexity of thought was not among the number; and the confusion of his ideas can neither be urged as an apology for himself, nor as a consolation to his readers.

Another paper from his pen, a Theory of Rain, appeared also in the first volume of the Edinburgh Transactions. He had long studied meteorology with great attention; and this communication contains one of the few speculations in that branch of knowledge entitled to the name of theory.

Dr Hutton begins with supposing that the quantity of humidity which air is capable of dissolving, increases with its temperature. Now, this increase must either be in the same ratio with the increase of heat, in a less ratio, or in a greater: in other words, for equal increments of heat, the increments of humidity must either constitute a series of which all the terms are equal to one another, or a series in which the terms continually decrease, or

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one in which they continually increase. * If either of the two first laws was that which took place in nature, a mixture of two portions of air, though each contained as much humidity as it was capable of dissolving, would never produce a condensation of that humidity. According to the first law, the temperature, the humidity, and the power of containing humidity, in the mixture, being all arithmetical means between the same quantities, as they existed previously to the mixture, the temperature produced would be exactly that which was required by the humidity to preserve it in its invisible form. If the second law took place, the moisture actually contained in the mixture would be less than the temperature was capable of supporting; so that, in

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 some places faster, and in others slower, 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 if 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.

stead of a condensation of humidity, the air would become drier than before.

If, on the other hand, the third law be that which takes place, after the mixture of two portions of air of different temperatures, the humidity will

be

greater than the temperature is able to maintain, and therefore a condensation of it will follow. Now, the experience of every day proves, that the mixture of two portions of humid air of unequal temperatures, does indeed produce a condensation of moisture, and therefore we are authorized to conclude that the last-mentioned law is that which actually prevails. *

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, show that there is great reason for supposing that the air has no chemical action whatever on the aqueous vapour contained in it.-Manchester Memoirs, Vol. V. p. 538.

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