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Heat is light in complete repose. Light is heat in rapid motion. Directly light is combined with a body, it becomes heat; but when it is thrown off from that body, it again becomes light.161

Whether this is true or false, we cannot tell; and many years, perhaps many generations, will have to elapse, before we shall be able to tell. But the service rendered by Leslie is quite independent of the accuracy of his opinion, as to the manner in which light and heat are interchanged. That they are interchanged, is the essential and paramount idea. And we must remember, that he made this idea the basis of his researches, at a period when some very important facts, or, I should rather say, some very conspicuous facts, were opposed to it; while the main facts which favoured it were still unknown. When he composed his work, the analogies between light and heat, with which we are now acquainted, had not been discovered; no one being aware, that double refraction, polarization, and other curious properties, are common to both. To grasp so wide a truth in the face of such obstacles, was a rare stroke of sagacity. But, on account of the obstacles, the inductive mind of England refused to receive the truth, as it was not generalized from a survey of all the facts. And Leslie, unfortunately for himself, died too soon to enjoy the exquisite pleasure of witnessing the empirical corroboration of his doctrine by direct experiment, although he clearly perceived, that the march of discovery, in reference to polarization, was leading the scientific world to a point, of which his keen eye had discerned the nature, when, to others, it was an almost invisible speck, dim in the distant offing.162

p. 162.

161"Heat is only light in the state of combination." Leslie on Heat, "Heat in the state of emission constitutes light." p. 174. "It is, therefore, the same subtle matter, that, according to its different modes of existence, constitutes either heat or light. Projected with rapid celerity, it forms light; in the state of combination with bodies it acts as heat.' p. 188. See also p. 403, "different states of the same identical substance." 162 In 1814, that is ten years after his great work was published, and about twenty years after it was begun, he writes from Paris: "My book on heat is better known" here "than in England. I was even reminded

In regard to the method adopted by Leslie, he assures us, that, in assuming the principles from which he reasoned, he derived great aid from poetry; for he knew that the poets are, after their own manner, consummate observers, and that their united observations form a treasury of truths, which are nowise inferior to the truths of science, and of which science must either avail herself, or else suffer from neglecting them.163 To apply these truths rightly, and to fit them to the exigencies of physical inquiry, is, no doubt, a most difficult task, since it involves nothing less than holding the balance between the conflicting claims of the emotions and the understanding. Like all great enterprises, it is full of danger, and, if undertaken by an ordinary mind, would certainly fail. But there are two circumstances which make it less dangerous in our time, than in any earlier period. The first circumstance is, that the supremacy of the human understanding, and its right to judge all theories for itself, is now more generally admitted than ever; so that there can be little fear of our leaning to the opposite side, and allowing poetry to encroach on science. The other circumstance is, that our knowledge of the laws of nature is much greater than that possessed by any previous age; and there is, consequently, less risk

of some passages in it which in England were considered as fanciful, but which the recent discoveries on the polarity of light have confirmed." Napier's Memoirs of Leslie, p. 28, prefixed to Leslie's Philosophical Treatises, edit. Edinb. 1838. Leslie died in 1832 (p. 40); and the decisive experiments of Forbes and Melloni were made between 1834 and 1836.

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163"The easiest mode of conceiving the subject, is to consider the heat that permeates all bodies, and unites with them in various proportions, as merely the subtle fluid of light in a state of combination. When forcibly discharged, or suddenly elicited from any substance, it again resumes its radiant splendour." "The same notion was embraced by the poets, and gives sublimity to their finest odes." ....“Those poetical images which have descended to our own times, were hence founded on a close observation of nature. Modern philosophy need not disdain to adopt them, and has only to expand and reduce to precision the original conceptions." Leslie's Treatises on Philosophy, pp. 308, 309. Again, at p. 416: This is not the first occasion in which we have to admire, through the veil of poetical imagery, the sagacity and penetration of those early sages. It would be weakness to expect nice conclusions in the infancy of science; but it is arrogant presumption to regard all the efforts of unaided genius with disdain."

of the imagination leading us into error, inasmuch as we have a large number of well-ascertained truths, which we can confront with every speculation, no matter how plausible or ingenious it may appear.

On both these grounds, Leslie was, I apprehend, justified in taking the course which he did. At all events, it is certain, that, by following it, he came nearer than would otherwise have been possible, to the conceptions of the most advanced scientific thinkers of our day. He distinctly recognized that, in the material world, there is neither break nor pause; so that what we call the divisions of nature have no existence, except in our minds.164 He was even almost prepared to do away with that imaginary difference between the organic and inorganic world, which still troubles many of our physicists, and prevents them from comprehending the unity and uninterrupted march of affairs. They, with their old notions of inanimate matter, are unable to see that all matter is living, and that what we term death is a mere expression by which we signify a fresh form of life. Towards this conclusion, all our knowledge is now converging; and it is certainly no small merit in Leslie, that he, sixty years ago, when really comprehensive views, embracing the whole creation, were scarcely known among scientific men, should have strongly insisted that all forces are of the same kind, and that we have no right to distinguish between them, as if some were living, and others were dead.165

We owe much to him, by whom such views were advocated. But they were then, and, in a certain,

164 "We should recollect that, in all her productions, Nature exhibits a chain of perpetual gradation, and that the systematic divisions and limitations are entirely artificial, and designed merely to assist the memory and facilitate our conceptions." Leslie on Heat, p. 506.

165 All forces are radically of the same kind, and the distinction of them into living and dead is not grounded on just principles." Leslie on Heat, p. 133. Compare p. 299: We shall perhaps find, that this prejudice, like many others, has some semblance of truth; and that even dead or inorganic substances must, in their recondite arrangements, exert such varying energies, and so like sensation itself, as if fully unveiled to our eyes, could not fail to strike us with wonder and surprise."

though far smaller, degree, they are now, so out of the domain of physical experience, that Leslie never could have obtained them by generalizing in the way which the inductive philosophy enjoins. His great work on heat was executed, as well as conceived, on the opposite plan;166 and his prejudices on this point were so strong, that we are assured by his biographer, that he would allow no merit to Bacon, who organized the inductive method into a system, and to whose authority we in England pay a willing, and I had almost said a servile, homage.167

Another curious illustration of the skill with which the Scotch mind, when once possessed of a principle, worked from it deductively, appears in the geological speculations of Hutton, late in the eighteenth century. It is well known, that the two great powers which have altered the condition of our planet, and made it what it is, are fire and water. Each has played so considerable a part, that we can hardly measure their relative importance. Judging, however, from the present appearance of the crust of the earth, there is reason to believe, that the older rocks are chiefly the result of fusion, and that the younger are aqueous deposits. It is, therefore, not unlikely, that, in the order in which the energies of nature have unfolded themselves, fire preceded water, and was its necessary precursor.168 But, all that we are as yet jus

167 66

166 Mr. Napier, in his Life of Leslie, p. 17, says of it, very gravely, “It's hypotheses are not warranted by the sober maxims of inductive logic." 'Notwithstanding the contrary testimony, explicitly recorded by the founders of the English experimental school, he denied all merit and influence to the immortal delineator of the inductive logic." Napier's Life of Leslie, p. 42.

168 The supposition, that volcanic agencies were formerly more potent than they are now, is by no means inconsistent with the scientific doctrine of uniformity, though it is generally considered to be so. It is one thing to assert the uniformity of natural laws; it is quite another thing to assert the uniformity of natural causes. Heat may once have produced far greater effects than it can do at present, and yet the laws of nature be unchanged, and the order and sequence of events unbroken. What I would venture to suggest to geologists is, that they have not taken sufficiently into account the theory of the interchange of forces, which seems to offer a solution of at least part of the problem. For, by that theory, a large portion of the heat which formerly existed may have been metamorphosed into other forces,

tified in asserting is, that these two causes, the igneous and the aqueous, were in full operation long before man existed, and are still busily working. Perhaps they are preparing another change in our habitation, suitable to new forms of life, as superior to man, as man is superior to the beings who occupied the earth before his time. Be this as it may, fire and water are the two most important and most general principles with which geologists are concerned; and though, on a superficial view, each is extremely destructive, it is certain that they can really destroy nothing, but can only decompose and recompose; shifting the arrangements of nature, but leaving nature herself intact. Whether one of these elements will ever again get the upper hand of its opponent, is a speculation of extreme interest. For, there is reason to suspect, that, at one period, fire was more active than water, and that, at another period, water was more active than fire. That they are engaged in incessant warfare, is a fact with which geologists are perfectly familiar, though, in this, as in many other cases, the poets were the first to discern the truth. To the eye of the geolo

such as light, chemical affinity, and gravitation. The increase of these forces consequent on the diminution of heat, would have facilitated the consolidation of matter; and until such forces possessed a certain energy, water, which afterwards became so prominent, could not have been formed. If the power of chemical affinity, for instance, were much weaker than it is, water would assuredly resolve itself into its component gases. Without wishing to lay too much stress on this speculation, I submit it to the consideration of competent judges, because I am convinced that any hypothesis, not absolutely inconsistent with the known laws of nature, is preferable to that dogma of interference, which what may be called the miraculous school of geologists wish to foist upon us, in utter ignorance of its incompatibility with the conclusions of the most advanced minds in other departments of thought.

The remarks in Sir Roderick Murchison's great work (Siluria, London, 1854, pp. 475, 476) on the "grander intensity of former causation," and on the difficulty this opposes to the "uniformitarians," apply merely to those who take for granted that each force has always been equally powerful: they do not affect those who suppose that it is only the aggregate of force which remains unimpaired. Though the distribution of forces may be altered, their gross amount is not susceptible of change, so far as the highest conceptions of our actual science extend. Consequently, there is no need for us to believe that, in different periods, the intensity of causation varies; though we may believe that some one agent, such as heat, had at one time more energy than it has ever had since.

VOL. II.

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