Imágenes de página
PDF
ePub

only difference between the two great masters. In the most important points, namely their method, they were entirely agreed. Both were essentially one-sided; both paid a too exclusive attention to one of the two principal agents which have altered, and are still altering, the crust of the earth; both reasoned from those agents, instead of reasoning to them; and both constructed their system without sufficiently studying the actual and existing facts; committing, in this respect, an error which the English geologists were the first to rectify.

As I am writing a history, not of science, but of scientific method, I can only briefly glance at the nature of those services which Hutton rendered to geology, and which are so considerable, that his system has been called its present basis.179 This, however, is too strongly expressed; for, though Hutton was far from denying the influence of water, 180 he did not concede enough to it, and there is a tendency among several geologists to admit that the system of Werner, considered as an aqueous theory, contains a larger amount of truth than the advocates of the igneous theory are willing to allow. Still, what Hutton did was most remarkable, especially in reference to what are now termed metamorphic rocks, the theory of whose formation he was the first to conceive.181 Into this, and into their connexion, on the one hand, with the sedimentary rocks, and, on the other hand, with those rocks whose origin is perhaps purely igneous, I could not enter without treading on debatable ground. But, putting aside what is yet uncertain, I will mention two circumstances respecting Hutton which are undisputed, and which will give some idea of his method, and

179"Has not only supplanted that of Werner, but has formed the foundation of the researches and writings of our most enlightened observers, and is justly regarded as the basis of all sound geology at the present day." Richardson's Geology, London, 1851, p. 38.

180 Hutton's Theory of the Earth, Edinb. 1795, vol. i. pp. 34, 41, 192, 290, 291, 593, vol. ii. pp. 236, 369, 378, 555.

181 In his writings, and in those of his illustrator, Playfair, we find the germ of the metamorphic theory." Lyell's Manual of Geology, London, 1851, p. 92.

of the turn of his mind. The first circumstance is, that, although he ascribed to subterranean heat, as exhibited in volcanic action, a greater and more constant energy than any previous inquirers had ventured to do,182 he preferred speculating on the probable consequences of that action, rather than drawing inferences from the facts which the action presented; he being on this point so indifferent, that he arrived at his conclusions without inspecting even a single region of active volcanos, where he might have watched the workings of nature, and seen what she was really about.183 The other circumstance is equally characteristic. Hutton, in his speculations concerning the geological effects of heat, naturally availed himself of the laws which Black had unfolded. One of those laws was, that certain earths owe their fusibility to the presence of fixed air in them before heat has expelled it; so that, if it were possible to force them to retain their fixed air, or carbonic acid gas, as we now call it, no amount of heat could deprive them of the capability of being fused. The fertile mind of Hutton saw, in this discovery, a principle from which he could construct a geological argument. It occurred to him, that great pressure would prevent the escape of fixed air from heated rocks, and would thus enable them to be fused, notwith

152 The shortest summary of this view is in his Theory of the Earth, Edin. 1795, vol. ii. p. 556. "The doctrine, therefore, of our Theory is briefly this; That whatever may have been the operation of dissolving water, and the chemical action of it upon the materials accumulated at the bottom of the sea, the general solidity of that mass of earth, and the placing of it in the atmosphere above the surface of the sea, has been the immediate operation of fire or heat melting and expanding bodies."

183 66

Although Hutton had never explored any region of active volcanos, he had convinced himself that basalt and many other trap rocks were of igneous origin." Lyell's Principles of Geology, London, 1853, p. 51. To this I may add, that he wrote his work without having examined granite. He says (Theory of the Earth, vol. i. p. 214), “It is true, I met with it on my return by the east coast, when I just saw it, and no more, at Peterhead and Aberdeen; but that was all the granite I had ever seen when I wrote my Theory of the Earth. I have, since that time, seen it in different places; because I went on purpose to examine it, as I shall have occasion to describe in the course of this work." Hutton's theory of granite is noticed in Bakewell's Geology, Lond. 1838, p. 101; but Mr. Bakewell does not seem to be aware that the theory was formed before the observations were made.

standing their elevated temperature. He then supposed that, at a period anterior to the existence of man, such a process had taken place under the surface of the sea, and that the weight of so great a column of water had prevented the rocks from being decomposed while they were subjected to the action of fire. In this way, their volatile parts were held together, and they themselves might be melted, which could not have happened except for this enormous pressure. By following this line of argument, he accounted for the consolidation of strata by heat; since, according to the premisses from which he started, the oily, or bituminous parts, would remain, in spite of the efforts of heat to disperse them.184 This striking speculation led to the inference, that the volatile components of a substance, and its fixed components, may be made to cohere, in the very teeth of that apparently irresistible agent whose business it is to effect their separation. Such an inference was contrary to all experience; or, to say the least, no man had ever seen an instance of it.185 Indeed, the event was only supposed to happen in consequence of circumstances which were never met with on the surface of the globe, and which, therefore, were out of the range of all human observation.186 The utmost that could be expected was, that, by means of our instruments, we might, perhaps, on a small scale,

184 Huttonian Theory, in Playfair, vol. i. pp. 38-40, 509, 510. Compare Playfair's Life of Hutton, p. 61.

185 Hence, the objections of Kirwan were invalid; because his argument against Hutton was grounded on experiments, where that very separation of the volatile and fixed parts takes place, which it excluded in that hypothesis of subterraneous heat." Huttonian Theory, in Playfair, vol. i. p. 193, Edinb. 1822.

186 Hutton says (Theory of the Earth, Edinb. 1795, vol. i. p. 94), “The place of mineral operations is not on the surface of the earth; and we are not to limit nature with our imbecility, or estimate the powers of nature by the measure of our own." See also p. 159, "mineral operations proper to the lower regions of the earth." And p. 527, "The mineral operations of nature lie in a part of the globe which is necessarily inaccessible to man, and where the powers of nature act under very different conditions from those which we find take place in the only situation where we can live." Again, in vol. ii. p. 97, "The present Theory of the Earth holds for principle that the strata are consolidated in the mineral regions far beyond the reach of human observation." Similarly, vol. ii. p. 484, "we judge not of the progress of things from the actual operations of the surface."

imitate the process which Hutton had imagined. It was possible, that a direct experiment might artificially combine great pressure with great heat, and that the result might be, that the senses would realize what the intellect had conceived.187 But the experiment had never been tried, and Hutton, who delighted in reasoning from ideas rather than from facts, was not likely to undertake it.188 He cast his speculation on the world, and left it to its fate.189 Fortunately, however, for the reception of his system, a very ingenious and skilful experimenter of that day, Sir James Hall, determined to test the speculation by an appeal to facts; and as nature did not supply the facts which he wanted, he created them for himself. He applied heat to powdered chalk, while, at the same time, with great delicacy of manipulation, he subjected the chalk to a pressure about equal to the weight of a column of water half a mile high. The result was, that, under that pressure, the volatile parts of the chalk were held together; the carbonic acid gas was unable to escape; the generation of quicklime was stopped; the ordinary operations of nature were baffled, and the whole composition, being preserved in its integrity, was fused, and, on

187 Hutton, however, did not believe that this could be done. "In the Theory of the Earth which was published, I was anxious to warn the reader against the notion that subterraneous heat and fusion could be compared with that which we induce by our chemical operations on mineral substances here upon the surface of the earth.” Hutton's Theory of the Earth, vol. i. p. 251.

188 See, in the Life of Hutton, in Playfair's Works, vol. iv. p. 62 note, a curious remark on his indifference to experimental verification. Innumerable passages in his work indicate this tendency, and show his desire to reason immediately from general principles. Thus, in vol. i. p. 17, “Let us strictly examine our principles in order to avoid fallacy in our reasoning." "We are now, in reasoning from principles, come to a point decisive of the question." vol. i. p. 177. "Let us now reason from our principles." vol. ii. p. 308. Hence, his constantly expressed contempt for experience; as in vol. ii. p. 367, where he says that we must "overcome those prejudices which contracted views of nature and magnified opinions of the experience of man may have begotten."

199 Playfair (Life of Hutton, p. 64) says that it drew "their attention" (i. e. the attention of "men of science"), " very slowly, so that several years elapsed before any one showed himself publicly concerned about it, either as an enemy or a friend." He adds, as one of the reasons of this, that it contained "too little detail of facts for a system which involved so much that was new, and opposite to the opinions generally received."

subsequently cooling, actually crystallized into solid marble.190 Never was triumph more complete. Never did a fact more fully confirm an idea.191 But, in the mind of Hutton, the idea preceded the fact by a long interval; since, before the fact was known, the theory had been raised, and the system which was built upon it had, indeed, been published several years. It, therefore, appears that one of the chief parts of the Huttonian Theory, and certainly its most successful part, was conceived in opposition to all preceding experience; that it pre-supposed a combination of events which no one had ever observed, and the mere possibility of which nothing but artificial experiment could prove; and, finally, that Hutton was so confident of the validity of his own method of inquiry, that he disdained to make the experiment himself, but left to another mind that empirical branch of the investigation which he deemed of little moment, but which we, in England, are taught to believe is the only safe foundation of physical research.192

190 The account of these experiments was read before the Royal Society of Edinburgh in 1805, and is printed in their Transactions, vol. vi. pp. 71185, Edinb. 1812, 4to. The general result was (pp. 148, 149), "That a pressure of 52 atmospheres, or 1700 feet of sea, is capable of forming a limestone in a proper heat; That under 86 atmospheres, answering nearly to 3000 feet, or about half a mile, a complete marble may be formed; and lastly, That, with a pressure of 173 atmospheres, or 5700 feet, that is little more than one mile of sea, the carbonate of lime is made to undergo complete fusion, and to act powerfully on other earths." See also p. 160: The carbonic acid of limestone cannot be constrained in heat by a pressure less than that of 1708 feet of sea." There is a short, and not very accurate, notice of these instructive experiments in Bakewell's Geology, London, 1838, pp. 249, 250.

191 As Sir James Hall says, "The truth of the most doubtful principle which Dr. Hutton has assumed, has thus been established by direct experiment." Transactions of the Royal Society of Edinburgh, vol. vi. p. 175.

192 See the remarks of Sir James Hall, in Transactions, vol. vi. pp. 74, 75. He observes that Hutton's "system, however, involves so many suppositions, apparently in contradiction to common experience, which meet us on the very threshold, that most men have hitherto been deterred from an investigation of its principles, and only a few individuals have justly appreciated its merits." . . . . "I conceived that the chemical effects ascribed by him to compression, ought, in the first place, to be investigated." It occurred to me that this principle was susceptible of being established in a direct manner by experiment, and I urged him to make the attempt; but he always rejected this proposal, on account of the immensity of the natural agents, whose operation he supposed to lie far beyond the

« AnteriorContinuar »