History of Civilization in England, Vol. 3 of 3

If, therefore, Black had done nothing more than set the example of a great physical philosopher giving free scope to the imagination, he would have conferred upon us a boon, the magnitude of which it is not easy to overrate. And it is very remarkable, that, before he died, that department of inorganic physics, which he cultivated with such success, was taken up by another eminent Scotchman, who pursued exactly the same plan, though with somewhat inferior genius. I allude, of course, to Leslie, whose researches on heat are well known to those who are occupied with this subject; while, for our present purpose, they are chiefly interesting as illustrating that peculiar method which, in the eighteenth century, seemed essential to the Scotch mind.

About thirty years after Black propounded his famous theory of heat, Leslie began to investigate the same topic, and, in 1804, published a special dissertation upon it.769 In that work, and in some papers in his Treatises on Philosophy, are contained his views, several of which are now known to be inaccurate,770 though some are of sufficient value to mark an epoch in the history of science. Such was his generalization respecting the connexion between the radiation of heat, and its reflection; bodies which reflect it most, radiating it least, and those which radiate it most, reflecting it least. Such, too, was another wide conclusion, which the best inquirers have since confirmed, namely, that, while heat is radiating from a body, the intensity of each ray is as the sine of the angle which it makes with the surface of that body.

These were important steps, and they were the result of experiments, preceded by large and judicious hypotheses. In relation, however, to the economy of nature, considered as a whole, they are of small account in comparison with what Leslie effected towards consolidating the great idea of light and heat being identical, and thus preparing his contemporaries for that theory of the interchange of forces, which is the capital intellectual achievement of the nineteenth century. But it is interesting to observe, that, with all his ardour, he could not go beyond a certain length. He was so hampered by the material tendencies of his time, that he could not bring himself to conceive heat as a purely supersensual force, of which temperature was the external manifestation.771 For this, the age was barely ripe. We accordingly find him asserting, that heat is an elastic fluid, extremely subtle, but still a fluid.772 His real merit was, that, notwithstanding the difficulties which beset his path, he firmly seized the great truth, that there is no fundamental difference between light and heat. As he puts it, each is merely a metamorphosis of the other. 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.773

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.774

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.775 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 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.776 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.777

We owe much to him, by whom such views were advocated. But they were then, and in a certain, 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;778 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.779

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.780 But, all that we are as yet justified 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 geologist, water is constantly labouring to reduce all the inequalities of the earth to a single level; while fire, with its volcanic action, is equally busy in restoring those inequalities, by throwing up matter to the surface, and in various ways disturbing the crust of the globe.781 And as the beauty of the material world mainly depends on that irregularity of aspect, without which scenery would have presented no variety of form, and but little variety of colour, we shall, I think, not be guilty of too refined a subtlety, if we say that fire, by saving us from the monotony to which water would have condemned us, has been the remote cause of that development of the imagination which has given us our poetry, our painting, and our sculpture, and has thereby not only wonderfully increased the pleasures of life, but has imparted to the human mind a completeness of function, to which, in the absence of such a stimulus, it could not have attained.

When geologists began to study the laws according to which fire and water had altered the structure of the earth, two different courses were open to them, namely, the inductive and the deductive. The deductive plan was to compute the probable consequences of fire and water, by reasoning from the sciences of thermotics and hydrodynamics; tracking each element by an independent line of argument, and afterwards coördinating into a single scheme the results which had been separately obtained. It would then only remain to inquire, how far this imaginary scheme harmonized with the actual state of things; and if the discrepancy between the ideal and the actual were not greater than might fairly be expected from the perturbations produced by other causes, the ratiocination would be complete, and geology would, in its inorganic department, become a deductive science. That our knowledge is ripe for such a process, I am far, indeed, from supposing; but this is the path which a deductive mind would take, so far as it was able. On the other hand, an inductive mind, instead of beginning with fire and water, would begin with the effects which fire and water had produced, and would first study these two agents, not in their own separate sciences, but in their united action as exhibited on the crust of the earth. An inquirer of this sort would assume, that the best way of arriving at truth would be to proceed from effects to causes, observing what had actually happened, and rising from the complex results up to a knowledge of the simple agents, by whose power the results have been brought about.

If the reader has followed the train of thought which I have endeavoured to establish in this chapter, and in the first volume, he will be prepared to expect that when, in the latter half of the eighteenth century, geology was first seriously studied, the inductive plan of proceeding from effects to causes became the favourite one in England; while the deductive plan of proceeding from causes to effects, was adopted in Scotland and in Germany. And such was really the case. It is generally admitted, that, in England, scientific geology owes its origin to William Smith, whose mind was singularly averse to system, and who, believing that the best way of understanding former causes was to study present effects, occupied himself, between the years 1790 and 1815, in a laborious examination of different strata.782 In 1815, he, after traversing the whole of England on foot, published the first complete geological map which ever appeared, and thus took the first great step towards accumulating the materials for an inductive generalization.783 In 1807, and, therefore, before he had brought his arduous task to an end, there was formed in London the Geological Society, the express object of which, we are assured, was, to observe the condition of the earth, but by no means to generalize the causes which had produced that condition.784 The resolution was, perhaps, a wise one. At all events, it was highly characteristic of the sober and patient spirit of the English intellect. With what energy and unsparing toil it has been executed, and how the most eminent members of the Geological Society have, in the pursuit of truth, not only explored every part of Europe, but examined the shell of the earth in America and in Northern Asia, is well known to all who are interested in these matters; nor can it be denied, that the great works of Lyell and Murchison prove that the men who are capable of such laborious enterprises, are also capable of the still more difficult achievement of generalizing their facts and refining them into ideas. They did not go as mere observers, but they went with the noble object of making their observations subservient to a discovery of the laws of nature. That was their aim; and all honour be to them for it. Still, it is evident, that their process is essentially inductive; it is a procedure from the observation of complex phenomena, up to the elements to which those phenomena are owing; it is, in other words, a study of natural effects, in order to learn the operation of natural causes.

Very different was the process in Germany and Scotland. In 1787, that is, only three years before William Smith began his labours, Werner, by his work on the classification of mountains, laid the foundation of the German school of geology.785 His influence was immense; and among his pupils we find the names of Mohs, Raumer, and Von Buch, and even that of Alexander Humboldt.786 But the geological theory which he propounded, depended entirely on a chain of argument from cause to effect. He assumed, that all the great changes through which the earth had passed, were due to the action of water. Taking this for granted, he reasoned deductively from premisses with which his knowledge of water supplied him. Without entering into details respecting his system, it is enough to say, that, according to it, there was originally one vast and primeval sea, which, in the course of time, deposited the primitive rocks. The base of all was granite; then gneiss; and others followed in their order. In the bosom of the water, which at first was tranquil, agitations gradually arose, which, destroying part of the earliest deposits, gave birth to new rocks, formed out of their ruins. The stratified thus succeeded to the unstratified, and something like variety was established. Then came another period, in which the face of the waters, instead of being merely agitated, was convulsed by tempests, and, amid their play and collision, life was generated, and plants and animals sprung into existence. The vast solitude was slowly peopled, the sea gradually retired; and a foundation was laid for that epoch, during which man entered the scene, bringing with him the rudiments of order and of social improvement.787

These were the leading views of a system which, we must remember, exercised great sway in the scientific world, and won over to its side minds of considerable power. Erroneous and far-fetched though it was, it had the merit of calling attention to one of the two chief principles which have determined the present condition of our planet. It had the further merit of provoking a controversy, which was eminently serviceable to the interests of truth. For, the great enemy of knowledge is not error, but inertness. All that we want is discussion, and then we are sure to do well, no matter what our blunders may be. One error conflicts with another; each destroys its opponent, and truth is evolved. This is the course of the human mind, and it is from this point of view that the authors of new ideas, the proposers of new contrivances, and the originators of new heresies, are benefactors of their species. Whether they are right or wrong, is the least part of the question. They tend to excite the mind; they open up the faculties; they stimulate us to fresh inquiry; they place old subjects under new aspects; they disturb the public sloth; and they interrupt, rudely, but with most salutary effect, that love of routine, which, by inducing men to go grovelling on in the ways of their ancestors, stands in the path of every improvement, as a constant, an outlying, and, too often, a fatal obstacle.

The method adopted by Werner was evidently deductive, since he argued from a supposed cause, and reasoned from it to the effects. In that cause, he found his major premiss, and thence he worked downwards to his conclusion, until he reached the world of sense and of reality. He trusted in his one great idea, and he handled that idea with consummate skill. On that very account, did he pay less attention to existing facts. Had he chosen, he, like other men, could have collected them, and subjected them to an inductive generalization. But he preferred the opposite path. To reproach him with this is irrational; for, in his journey after truth, he chose one of the only two roads which are open to the human mind. In England, indeed, we are apt to take for granted that one road is infinitely preferable to the other. It may be so; but on this, as on many other subjects, assertions are current which have never been proved. At all events, Werner was so satisfied with his method, that he would not be at the pains of examining the position of rocks and their strata, as they are variously exhibited in different countries; he did not even explore his own country, but, confining himself to a corner of Germany, he began and completed his celebrated system, without investigating the facts on which, according to the inductive method, that system should have been built.788

Exactly the same process, on the same subject, and at the same time, was going on in Scotland. Hutton, who was the founder of Scotch geology, and who, in 1788, published his Theory of the Earth, conducted the inquiry just as Werner did; though, when he began his speculations, he had no knowledge of what Werner was doing.789 The only difference between them was, that while Werner reasoned from the agency of water, Hutton reasoned from the agency of fire. The cause of this may, I think, be explained. Hutton lived in a country where some of the most important laws of heat had, for the first time, been generalized, and where consequently, that department of inorganic physics had acquired great reputation. It was natural for a Scotchman to take more than ordinary interest in a subject in which Scotland had been so successful, and had obtained so much fame. We need not, therefore, wonder that Hutton, who, like all men, felt the intellectual bent of the time in which he lived, should have yielded to an influence of which he was, perhaps, unconscious. In obedience to the general mental habits of his country he adopted the deductive method. In further obedience to the more special circumstances connected with his own immediate pursuits, he gathered the principles from which he reasoned from a study of fire, instead of gathering them, as Werner did, from a study of water.

Hence it is, that, in the history of geology, the followers of Werner are known as Neptunists, and those of Hutton as Plutonists.790 And these terms represent the 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.791 This, however, is too strongly expressed; for, though Hutton was far from denying the influence of water,792 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.793 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 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,794 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 volcanoes, where he might have watched the workings of nature, and seen what she was really about.795 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, notwithstanding 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.796 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.797 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.798 The utmost that could be expected was, that, by means of our instruments, we might, perhaps, on a small scale, 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.799 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.800 He cast his speculation on the world, and left it to its fate.801 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 subsequently cooling, actually crystallized into solid marble.802 Never was triumph more complete. Never did a fact more fully confirm an idea.803 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.804