457. The time indeed was, and we are not yet far removed from it, when one of the most important principles involved in Dr Hutton's theory was not only unknown, but could not be discovered. This was before the causticity produced in limestone by exposure to fire was understood, and when it was not known that it arose from the expulsion of a certain aerial fluid, which before was a component part of the stone. It could not then be perceived, that this aerial part might be retained by pressure, even in spite of the action of fire, and that in a region where great compression existed, the absence of causticity was no proof that great heat had not been applied. The discoveries of Dr BLACK, therefore, mark an era, before which men were not qualified to judge of the nature of the powers that had acted in the consolidation of mineral substances. Those discoveries were, indeed, destined to produce a memorable change in chemistry, and in all the branches of knowledge allied to it; and have been the foundation of that brilliant progress, by which a collection of practical rules, and of insulated facts, has in a few years risen to the rank of a very perfect science. But even before they had explained the nature of carbonic gas, and its affinity to calcareous earth, I am not sure but that Dr Hutton's theory was, at least, partly formed, though it must certainly have remained, even in his own opinion, exposed to great difficulties. His active and penetrating genius soon perceived, in the experiments of his friend, the solution of those difficulties, and formed that happy combination of principles, which has enabled him to explain the most enigmatical appearances in the natural history of the earth. As we are not yet far removed from the time when our chemical knowledge was too imperfect to admit of a satisfactory explanation of the phenomena of mineralogy, so it is not unlikely that we are approaching to other discoveries that are to throw new light on this science. It would, however, be to argue strangely to say, that we must wait till those discoveries are made before we begin any theoretical reasonings. If this rule were followed, we should not know where the imperfections of our science lay, nor when the remedies were found out, should we be in a condition to avail ourselves of them. Such conduct would not be caution, but timidity, and an excess of prudence fatal to all philosophical inquiry.