History of Civilization in England, Vol. 3 of 3

by Henry Thomas Buckle

Among the sciences which concern the inorganic world, the laws of heat occupy a conspicuous place. On the one hand, they are connected with geology, being intimately allied, and, indeed, necessarily bound up, with every speculation respecting the changes and present condition of the crust of the earth. On the other hand, they touch the great questions of life, both animal and vegetable; they have to do with the theory of species, and of race; they modify soil, food, and organization; and to them we must look for valuable help towards solving those great problems in biology, which, of late years, have occupied the attention of the boldest and most advanced philosophers.

Our present knowledge of the laws of heat, may be briefly stated as branching into five fundamental divisions. These are: latent heat; specific heat; the conduction of heat; the radiation of heat; and, finally, the undulatory theory of heat; by which last, we are gradually discarding our old material views, and are accustoming ourselves to look upon heat as simply one of the forms of force, all of which, such as light, electricity, magnetism, motion, gravitation, and chemical affinity, are constantly assuming each other's shape, but, in their total amount, are incapable either of increase or of diminution.[756] This grand conception, which is now placing the indestructibility of force on the same ground as the indestructibility of matter, has an importance far above its scientific value, considerable as that undoubtedly is. For, by teaching us, that nothing perishes, but that, on the contrary, the slightest movement of the smallest body, in the remotest region, produces results which are perpetual, which diffuse themselves through all space, and which, though they may be metamorphosed, cannot be destroyed, it impresses us with such an exalted idea of the regular and compulsory march of physical affairs, as must eventually influence other and higher departments of inquiry. Our habits of thought are so connected and interwoven, that notions of law and of the necessary concatenation of things, can never be introduced into one field of speculation, without affecting other fields which lie contiguous to it. When, therefore, the modern doctrine of conservation of force,[757] becomes firmly coupled with the older doctrine of conservation of matter, we may rest assured that the human mind will not stop there, but will extend to the study of Man, inferences analogous to those already admitted in the study of Nature. Having once recognized that the condition of the material universe, at any one moment, is simply the result of every thing which has happened at all preceding moments, and that the most trivial disturbance would so violate the general scheme, as to render anarchy inevitable, and that, to sever from the total mass even the minutest fragment, would, by dislocating the structure, bury the whole in one common ruin, we, thus admitting the exquisite adjustment of the different parts, and discerning, too, in the very beauty and completeness of the design, the best proof that it has never been tampered with by the Divine Architect, who called it into being, in whose Omniscience both the plan, and the issue of the plan, resided with such clearness and unerring certainty, that not a stone in that superb and symmetrical edifice has been touched since the foundation of the edifice was laid, are, by ascending to this pitch and elevation of thought, most assuredly advancing towards that far higher step, which it will remain for our posterity to take, and which will raise their view to so commanding a height, as to insure the utter rejection of those old and eminently irreligious dogmas of supernatural interference with the affairs of life, which superstition has invented, and ignorance has bequeathed, and the present acceptance of which betokens the yet early condition of our knowledge, the penury of our intellectual resources, and the inveteracy of the prejudices in which we are still immersed.

It is, therefore, natural, that the physical doctrine of indestructibility applied to force as well as to matter, should be essentially a creation of the present century, notwithstanding a few allusions made to it by some earlier thinkers, all of whom, however, groped vaguely, and without general purpose. No preceding age was bold enough to embrace so magnificent a view as a whole, nor had any preceding philosophers sufficient acquaintance with nature to enable them to defend such a conception, even had they desired to entertain it. Thus, in the case now before us, it is evident, that while heat was believed to be material, it could not be conceived as a force, and therefore no one could grasp the theory of its metamorphosis into other forces; though there are passages in Bacon which prove that he wished to identify it with motion. It was first necessary to abstract heat into a mere property or affection of matter, and there was no chance of doing this until heat was better understood in its immediate antecedents, that is, until, by the aid of mathematics, its proximate laws had been generalized. But, with the single exception of Newton, whose efforts, notwithstanding his gigantic powers, were, on this subject, very unsatisfactory, and who, moreover, had a decided leaning towards the material theory, no one attempted to unravel the mathematical laws of heat till the latter half of the eighteenth century, when Lambert and Black began the career which Prevost and Fourier followed up. The mind, having been so slow in mastering the preliminaries and outworks of the inquiry, was not ripe for the far more difficult enterprise of idealizing heat itself, and so abstracting it, as to strip it of its material attributes, and leave it to nothing but the speculative notion of an immaterial force.

From these considerations, which were necessary to enable the reader to appreciate the value of what was done in Scotland, it will be seen how essential it was that the laws of the movement of heat should be studied before its nature was investigated, and before the emission theory could be so seriously attacked as to allow of the possibility of that great doctrine of the indestructibility of force, which, I make no doubt, is destined to revolutionize our habits of thought, and to give to future speculations a basis infinitely wider than any previously known. In regard to the movements of heat, we owe the laws of conduction and of radiation chiefly to France and Geneva, while the laws of specific heat, and those of latent heat, were discovered in Scotland. The doctrine of specific heat, though interesting, has not the scientific importance which belongs to the other departments of this great subject; but the doctrine of latent heat is extremely curious, not only in itself, but also on account of the analogies it suggests with various branches of physical inquiry.

What is termed latent heat, is exhibited in the following manner. If, in consequence of the application of heat, a solid passes into a liquid, as ice, for instance, into water, the conversion occupies a longer time than could be explained by any theory which had been propounded down to the middle of the eighteenth century. Neither was it possible to explain how it is, that ice never rises above the temperature of 32° until it is actually melted, no matter what the heat of the adjacent bodies may be. There were no means of accounting for these circumstances. And though practical men, being familiar with them, did not wonder at them, they caused great astonishment among thinkers, who were accustomed to analyze events, and to seek a reason for common and every-day occurrences.

Soon after the middle of the eighteenth century, Black, who was then one of the professors in the University of Glasgow, turned his attention to this subject.[758] He struck out a theory which, being eminently original, was violently attacked, but is now generally admitted. With a boldness and reach of thought not often equalled, he arrived at the conclusion, that whenever a body loses some of its consistence, as in the case of ice becoming water, or water becoming steam, such body receives an amount of heat which our senses, though aided by the most delicate thermometer, can never detect. For, this heat is absorbed; we lose all sight of it, and it produces no palpable effect on the material world, but becomes, as it were, a hidden property. Black, therefore, called it latent heat, because, though we conceive it as an idea, we cannot trace it as a fact. The body is, properly speaking, hotter; and yet its temperature does not rise. Directly, however, the foregoing process is inverted, that is to say, directly the steam is condensed into water, or the water hardened into ice, the heat returns into the world of sense; it ceases to be latent, and communicates itself to the surrounding objects. No new heat has been created; it has, indeed, appeared and disappeared, so far as our senses are concerned; but our senses were deceived, since there has, in truth, been neither addition nor diminution.[759] That this remarkable theory paved the way for the doctrine of the indestructibility of force, will be obvious to whoever has examined the manner in which, in the history of the human mind, scientific conceptions are generated. The process is always so slow, that no single discovery has ever been made, except by the united labours of several successive generations. In estimating, therefore, what each man has done, we must judge him, not by the errors he commits, but by the truths he propounds. Most of his errors are not really his own. He inherits them from his predecessors; and if he throws some of them off, we should be grateful, instead of being dissatisfied that he has not rejected all. Black, no doubt, fell into the error of regarding heat as a material substance, which obeys the laws of chemical composition.[760] But this was merely an hypothesis, which was bequeathed to him, and with which the existing state of thought forced him to encumber his theory. He inherited the hypothesis, and could not get rid of his troublesome possession. The real service which he rendered is, that, in spite of that hypothesis, which clung to him to the last, he, far more than any of his contemporaries, contributed towards the great conception of idealizing heat, and thus enabled his successors to admit it into the class of immaterial and supersensual forces. Once admitted into that class, the list of forces became complete; and it was comparatively easy to apply to the whole body of force, the same notion of indestructibility, which had previously been applied to the whole body of matter. But it was hardly possible to effect this object, while heat stood, as it were, midway between force and matter, yielding opposite results to different senses; amenable to the touch, but invisible to the eye. What was wanting, was to remove it altogether out of the jurisdiction of the senses, and to admit that, though we experience its effects, we can only conceive its existence. Towards accomplishing this, Black took a prodigious stride. Unconscious, perhaps, of the remote tendency of his own labours, he undermined that doctrine of material heat, which he seemed to support. For, by his advocacy of latent heat, he taught that its movements constantly baffle, not only some of our senses, but all of them; and that, while our feelings make us believe that heat is lost, our intellect makes us believe that it is not lost. Here, we have apparent destructibility, and real indestructibility. To assert that a body received heat without its temperature rising, was to make the understanding correct the touch, and defy its dictates. It was a bold and beautiful paradox, which required courage as well as insight to broach, and the reception of which marks an epoch in the human mind, because it was an immense step towards idealizing matter into force. Some, indeed, have spoken of invisible matter; but that is a contradiction in terms, which will never be admitted, as long as the forms of speech remain unchanged. Nothing can be invisible, except force, mind, and the Supreme Cause of all. We must, therefore, ascribe to Black the signal merit that he first, in the study of heat, impeached the authority of the senses, and thereby laid the foundation of every thing which was afterwards done. Besides the relation which his discovery bears to the indestructibility of force, it is also connected with one of the most splendid achievements effected by this generation in inorganic physics; namely, the establishment of the identity of light and heat. To the senses, light and heat, though in some respects similar, are in most respects dissimilar. Light, for instance, affects the eye, and not the touch. Heat affects the touch, but, under ordinary circumstances, does not affect the eye. The capital difference, however, between them is, that heat, unlike light, possesses the property of temperature; and this property is so characteristic, that until our understandings are invigorated by science, we cannot conceive heat separated from temperature, but are compelled to confuse one with the other. Directly, however, men began to adopt the method followed by Black, and were resolved to consider heat as supersensual, they entered the road which led to the discovery of light and heat being merely different developments of the same force. Ignoring the effects of heat on themselves, or on any part of the creation, which was capable of feeling its temperature, and would therefore be deceived by it, nothing was left for them to do, but to study its effects on the inanimate world. Then, all was revealed. The career of discovery was fairly opened; and analogies between light and heat, which even the boldest imagination had hardly suspected, were placed beyond a doubt. To the reflection of heat, which had been formerly known, were now added the refraction of heat, its double refraction, its polarization, its depolarization, its circular polarization, the interference of its rays, and their retardation; while, what is more remarkable than all, the march of our knowledge on these points was so swift, that before the year 1836 had come to a close, the chain of evidence was completed by the empirical investigations of Forbes and Melloni, they themselves little witting that every thing which they accomplished was prepared before they were born, that they were but the servants and followers of him who indicated the path in which they trod, and that their experiments, ingenious as they were, and full of resource, were simply the direct practical consequence of one of those magnificent ideas which Scotland has thrown upon the world, and the memory of which is almost enough so to bribe the judgment, as to tempt us to forget, that, while the leading intellects of the nation were engaged in such lofty pursuits, the nation itself, untouched by them, passed them over with cold and contemptuous indifference, being steeped in that deadening superstition, which turns a deaf ear to every sort of reason, and will not hearken to the voice of the charmer, charm he never so wisely.

By thus considering the descent and relationship of scientific conceptions, we can alone understand what we really owe to Black's discovery of latent heat. In regard to the method of the discovery, little need be said, since every student of the Baconian philosophy must see, that the discovery was of a kind for which none of the maxims of that system had provided. As latent heat escapes the senses, it could not obey the rules of a philosophy, which grounds all truth on observation and experiment. The subject of the inquiry being supersensual, there was no scope for what Bacon called crucial experiments and separations of nature. The truth was in the idea; experiments, therefore, might illustrate it, might bring it up to the surface, and so enable men to grasp it, but could not prove it. And this, which appears on the very face of the discovery, is confirmed by the express testimony of Dr. Thomson, who knew Black, and was, indeed, one of the most eminent of his pupils. We are assured by this unimpeachable witness, that Black, about the year 1759 began to speculate concerning heat; that the result of those speculations was the theory of latent heat; that he publicly taught that theory in the year 1761; but that the experiments which were necessary to convince the world of it were not made till 1764,[761] though, as I need hardly add, according to the inductive method, it was a breach of all the rules of philosophy to be satisfied with the theory three years before the experiments were made, and it was a still greater breach, not only to be satisfied with it, but to have openly promulgated it as an original and unquestionable truth, which explained, in a new manner, the economy of the material world.

The intellect of Black belonged to a class, which, in the eighteenth century, was almost universal in Scotland, but was hardly to be found in England, and which, for want of a better word, we are compelled to call deductive, though fully admitting that even the most deductive minds have in them a large amount of induction, since, indeed, without induction, the common business of life could not be carried on. But for the purposes of scientific classification, we may say, that a man or an age is deductive, when the favourite process is reasoning from principles instead of reasoning to them, and when there is a tendency to underrate the value of specific experience. That this was the case with the illustrious discoverer of latent heat, we have seen, both from the nature of the discovery, and from the decisive testimony of his friend and pupil. And a further confirmation may be found in the circumstance, that, having once propounded his great idea, he, instead of instituting a long series of laborious experiments, by which it might be verified in its different branches, preferred reasoning from it according to the general maxims of dialectic; pushing it to its logical consequences, rather than tracking it into regions where the senses might either confirm or refute it.[762] By following this process of thought, he was led to some beautiful speculations, which are so remote from experience, that even now, with all the additional resources of our knowledge, we cannot tell whether they are true or false. Of this kind were his views respecting the causes of the preservation of man, whose existence would, he thought, be endangered, except for the power which heat possesses of lying latent and unobserved. Thus, for example, when a long and severe winter was followed by sudden warmth, it appeared natural that the ice and snow should melt with corresponding suddenness; and if this were to happen, the result would be such terrible inundations, that it would be hardly possible for man to escape from their ravages. Even if he escaped, his works, that is, the material products of his civilization, would perish. From this catastrophe, nothing saves him but the latent power of heat. Owing to this power, directly the ice and snow begin to melt at their surface, the heat enters their structure, where a large part of it remains in abeyance, and thus losing much of its power, the process of liquefaction is arrested. This dreadful agent is lulled, and becomes dormant. It is weakened at the outset of its career, and is laid up, as in a storehouse, from which it can afterwards emerge, gradually, and with safety to the human species.[763]

In this way, as summer advances, a vast magazine of heat is accumulated, and is preserved in the midst of water, where it can do man no injury, since, indeed, his senses are unable to feel it. There the heat remains buried, until, in the rotation of the seasons, winter returns, and the waters are congealed into ice. In the process of congelation, that treasury of heat, which had been hidden all the summer, reappears; it ceases to be latent; and now, for the first time, striking the senses of man, it tempers, on his behalf, the severity of winter. The faster the water freezes, the faster the heat is disengaged; so that, by virtue of this great law of nature, cold actually generates warmth, and the inclemency of every season, though it cannot be hindered, is softened in proportion as the inclemency is more threatening.[764]