Chapter V (5)

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.

[758] He was appointed professor in 1756; and 'it was during his
residence in Glasgow, between the years 1759 and 1763, that he
brought to maturity those speculations concerning the combination
of heat with matter, which had frequently occupied a portion of
his thoughts.' _Thomson's History of Chemistry_, vol. i. pp. 319,
320.

[759] _Black's Lectures on Chemistry_, vol. i. pp. 116, 117; and in
various places. Dr. Robison, the editor of these Lectures, says,
p. 513, 'Nothing could be more simple than his doctrines of latent
heat. The experience of more than a century had made us consider
the thermometer as a sure and accurate indicator of heat, and of
all its variations. We had learned to distrust all others. Yet, in
the liquefaction and vaporization of bodies, we had proofs
uncontrovertible of the entrance of heat into the bodies. And we
could, by suitable processes, get it out of them again. Dr. Black
said that it was concealed in them,--_latent_,--it was as much
concealed as carbonic acid is in marble, or water in zeolite,--it
was concealed till Dr. Black detected it. He called it Latent
Heat. He did not mean by this term that it was a different kind of
heat from the heat which expanded bodies, but merely that it was
concealed from our sense of heat, and from the thermometer.' See
also p. xxxvii.: 'Philosophers had long been accustomed to
consider the thermometer as the surest means for detecting the
presence of heat or fire in bodies, and they distrusted all
others.'

[760] 'Fluidity is the consequence of a certain combination of calorific
matter with the substance of solid bodies,' &c. _Black's
Lectures_, vol. i. p. 133. Compare p. 192, and the remarks in
_Turner's Chemistry_, 1847, vol. i. p. 31, on Black's views of the
'chemical combination' of heat. Among the backward chemists, we
still find traces of the idea of heat obeying chemical laws.

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.

[761] 'So much was he convinced of this, that he taught the doctrine in
his lectures in 1761, before he had made a single experiment on
the subject.' ... 'The requisite experiments were first attempted
by Dr. Black in 1764.' _Thomson's History of Chemistry_, vol. i.
p. 324. See also pp. 319, 320; and on the history of the idea in
Black's mind as early as the year 1754, see the interesting
extracts from his note-books in Robison's appendix to _Black's
Lectures_, vol. i pp. 525, 526.

The statement of Dr. Thomson refers to the completion, or last
stage, of the discovery, namely the vaporific combination of heat.
But from a letter which Black wrote to Watt in 1780 (_Muirhead's
Life of Watt_, London, 1859, p. 303), it appears that Thomson has
even understated the question, and that Black, instead of first
teaching his theory in 1761, taught it three years earlier, that
is, _six_ years before the decisive experiments were made. 'I
began,' writes Black, 'to give the doctrine of latent heat in my
lectures at Glasgow in the winter 1757-58, which, I believe, was
the first winter of my lecturing there; or if I did not give it
that winter, I certainly gave it in the 1758-59; and I have
delivered it every year since that time in my winter lectures,
which I continued to give at Glasgow until winter 1766-67, when I
began to lecture in Edinburgh.'

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]

[762] And he distinctly states that, even in other matters, when he did
make experiments, their object was to confirm theory, and not to
suggest it. Thus, to give one of many instances, in his
_Lectures_, vol. i. p. 354, he says, respecting salts, 'When we
examine the solidity of this reasoning by an experiment, _we have
the pleasure to find facts agree exactly with the theory_.'

[763] See a good summary of this idea in _Black's Lectures on
Chemistry_, vol. i. p. 118. Contrasting his theory of heat with
that previously received, he says, 'But, were the ice and snow to
melt as suddenly as they must necessarily do, were the former
opinion of the action of heat in melting them well founded, the
torrents and inundations would be incomparably more irresistible
and dreadful. They would tear up and sweep away every thing, and
that so suddenly, that mankind should have great difficulty to
escape from their ravages.'

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]

[764] 'Dr. Black quickly perceived the vast importance of this
discovery; and took a pleasure in laying before his students a
view of the extensive and beneficial effects of this habitude of
heat in the economy of nature. He made them remark how, by this
means, there was accumulated, during the summer season, a vast
magazine of heat, which, by gradually emerging, during
congelation, from the water which covers the face of the earth,
serves to temper the deadly cold of winter. Were it not for this
quantity of heat, amounting to 145 degrees, which emerges from
every particle of water as it freezes, and which diffuses itself
through the atmosphere, the sun would no sooner go a few degrees
to the south of the equator, than we should feel all the horrors
of winter.' _Robison's Preface to Black's Lectures_, vol. i.
p. xxxviii.

Thus, again, inasmuch as heat becomes latent, and flies from the senses, not only when ice is passing into water, but also when water is passing into steam, we find in this latter circumstance, one of the reasons why man and other animals can live in the tropics, which, but for this, would be deserted. They are constantly suffering from the heat which is collected in their bodies, and which, considered by itself, is enough to destroy them. But this heat causes thirst, and they consequently swallow great quantities of fluid, much of which exudes through the pores of the skin in the form of vapour. And as, according to the theory of latent heat, vapour cannot be produced without a vast amount of heat being buried within it, such vapour absorbs and carries off from the body, that which, if left in the system, would prove fatal. To this we must add, that, in the tropics, the evaporation of water is necessarily rapid, and the vapour which is thus produced, becomes another storehouse of heat, and a vehicle by which it is removed from the earth, and prevented from unduly interfering with the economy of life.[765]

[765] As I am writing an account of Black's views, and not a criticism
of them, I shall give them, without comment, in his own words, and
in the words of one of his pupils. 'Here we can also trace another
magnificent train of changes, which are nicely accommodated to the
wants of the inhabitants of this globe. In the equatorial regions,
the oppressive heat of the sun is prevented from a destructive
accumulation by copious evaporation. The waters, stored with their
vaporific heat, are thus carried aloft into the atmosphere, till
the rarest of the vapour reaches the very cold regions of the air,
which immediately forms a small portion of it into a fleecy cloud.
This also further tempers the scorching heat by its opacity,
performing the acceptable office of a screen. From thence the
clouds are carried to the inland countries, to form the sources in
the mountains, which are to supply the numberless streams that
water the fields. And, by the steady operation of causes, which
are tolerably uniform, the greater part of the vapours pass on to
the circumpolar regions, there to descend in rains and dews; and
in this beneficent conversion into rain, by the cold of those
regions, each particle of steam gives up the 700 or 800 degrees of
heat which were latent in it. These are immediately diffused, and
soften the rigour of those less comfortable climates.' ... 'I am
persuaded that the heat absorbed in spontaneous evaporation
greatly contributes to enable animals to bear the heat of the
tropical climates, where the thermometer frequently continues to
show the temperature of the human body. Such heats, indeed, are
barely supportable, and enervate the animal, making it lazy and
indolent, indulging in the most relaxed postures, and avoiding
every exertion of body or mind. The inhabitants are induced to
drink large draughts of diluting liquors, which transude through
their pores most copiously, carrying off with them a vast deal of
this troublesome and exhausting heat. There is in the body itself
a continual laboratory, or manufacture of heat, and, were the
surrounding air of such a temperature as not to carry it off, it
would soon accumulate so as to destroy life. The excessive
perspiration, supplied by diluting draughts, performs the same
office as the cold air without the tropics, in guarding us from
this fatal accumulation.' _Black's Lectures_, vol. i. pp. xlvi.
214.

From these and many other arguments, all of which were so essentially speculative, and dealt with such hidden processes of nature, that even now we are not justified either in confidently admitting them or in positively denying them, Black was led to that great doctrine of the indestructibility of heat,[766] which, as I have pointed out, has, in its connexion with the indestructibility of force, a moral and social importance even superior to its scientific value. Though the evidence of which he was possessed was far more scanty than what we now have, he, by the reach of his commanding intellect, rather than by the number and accuracy of his facts, became so penetrated with a conviction of the stability of physical affairs, that he not only applied that idea to the subtle phenomena of heat, but, what was much harder to do, he applied it to cases in which heat so entirely escapes the senses, that man has no cognizance of it, except through the medium of the imagination. According to his view, heat passes through an immense variety of changes, during which it appears to be lost; changes which no eye can ever see, which no touch can ever experience, and which no instrument can ever measure. Still, and in the midst of all these changes, it remains intact. From it nothing can be taken, and to it nothing can be added. In one of those fine passages of his Lectures, which, badly reported as they are,[767] bear the impress of his elevated genius, Black, after stating what would probably happen, if the total amount of heat existing in the world were to be diminished, proceeds to speculate on the consequences of its being increased. Were it possible for any power to add to it ever so little, it would at once overstep its bounds; the equilibrium would be disturbed; the framework of affairs would be disjointed. The evil rapidly increasing, and acting with accumulated force, nothing would be able to stop its ravages. It must continue to gain ground, till all other principles are absorbed and conquered. Sweeping on, unhindered, and irresistible, before it, every animal must perish, the whole vegetable world must disappear, the waters must pass into vapour, and the solid parts of the globe be merged and melted, until, at length, the glorious fabric, loosened and dissolved, would fall away, and return to that original chaos out of which it had been evolved.[768]

[766] See his strong protest against the notion that heat is ever
destroyed, in his _Lectures_, vol. i. pp. 125, 126, 164, 165.

[767] They were published after his death from such scanty materials,
that their editor, Dr. Robison, says (_Preface to Black's
Lectures_, vol. i. p. x.): 'When I then entered seriously on the
task, I found that the notes were (with the exception of perhaps a
score of lectures) in the same imperfect condition that they had
been in from the beginning, consisting entirely of single leaves
of paper, in octavo, full of erasions, interlinings, and
alterations of every kind; so that, in many places, it was not
very certain which of several notes was to be chosen.'

[768] 'On the other hand, were the heat which at present cherishes and
enlivens this globe, allowed to increase beyond the bounds at
present prescribed to it; beside the destruction of all animal and
vegetable life, which would be the immediate and inevitable
consequence, the water would lose its present form, and assume
that of an elastic vapour like air; the solid parts of the globe
would be melted and confounded together, or mixed with the air and
water in smoke and vapour; and nature would return to the original
chaos.' _Black's Lectures_, vol. i. pp. 246, 247.

These, like many other of the speculations of this great thinker, will find small favour with those purely inductive philosophers, who not only suppose, perhaps rightly, that all our knowledge is in its beginning built upon facts, but who countenance, what seems to me, the very dangerous opinion, that every increase of knowledge must be preceded by an increase of facts. To such men it will appear, that Black had far better have occupied himself in making new observations, or devising new experiments, than in thus indulging his imagination in wild and unprofitable dreams. They will think, that these flights of fancy are suitable, indeed, to the poet, but unworthy of that severe accuracy, and of that close attention to facts, which ought to characterize a philosopher. In England, especially, there is, among physical inquirers, an avowed determination to separate philosophy from poetry, and to look upon them, not only as different, but as hostile. Among that class of thinkers, whose zeal and ability are beyond all praise, and to whom we owe almost unbounded obligations, there does undoubtedly exist a very strong opinion, that, in their own pursuit, the imagination is extremely dangerous, as leading to speculations, of which the basis is not yet assured, and generating a desire to catch too eagerly at distant glimpses before the intermediate ground has been traversed. That the imagination has this tendency is undeniable. But they who object to it on this account, and who would, therefore, divorce poetry from philosophy, have, I apprehend, taken a too limited view of the functions of the human mind, and of the manner in which truth is obtained. There is, in poetry, a divine and prophetic power, and an insight into the turn and aspect of things, which, if properly used, would make it the ally of science instead of the enemy. By the poet, nature is contemplated on the side of the emotions; by the man of science, on the side of the understanding. But the emotions are as much a part of us as the understanding: they are as truthful; they are as likely to be right. Though their view is different, it is not capricious. They obey fixed laws; they follow an orderly and uniform course; they run in sequences; they have their logic and method of inference. Poetry, therefore, is a part of philosophy, simply because the emotions are a part of the mind. If the man of science despises their teaching, so much the worse for him. He has only half his weapons; his arsenal is unfilled. Conquests, indeed, he may make, because his native strength may compensate the defects of his equipment. But his success would be more complete and more rapid, if he were properly furnished and made ready for the battle. And I cannot but regard as the worst intellectual symptom of this great country, what I must venture to call the imperfect education of physical philosophers, as exhibited both in their writings and in their trains of thought. This is the more serious, because they, as a body, form the most important class in England, whether we look at their ability, or at the benefits we have received from them, or at the influence they are exercising, and are likely to exercise, over the progress of society. It cannot, however, be concealed, that they display an inordinate respect for experiments, an undue love of minute detail, and a disposition to overrate the inventors of new instruments, and the discoverers of new, but often insignificant, facts. Their predecessors of the seventeenth century, by using hypotheses more boldly, and by indulging their imagination more frequently, did certainly effect greater things, in comparison with the then state of knowledge, than our contemporaries, with much superior resources, have been able to achieve. The magnificent generalizations of Newton and Harvey could never have been completed in an age absorbed in one unvarying round of experiments and observations. We are in that predicament, that our facts have outstripped our knowledge, and are now encumbering its march. The publications of our scientific institutions, and of our scientific authors, overflow with minute and countless details, which perplex the judgment, and which no memory can retain. In vain do we demand that they should be generalized, and reduced into order. Instead of that, the heap continues to swell. We want ideas, and we get more facts. We hear constantly of what nature is doing, but we rarely hear of what man is thinking. Owing to the indefatigable industry of this and the preceding century, we are in possession of a huge and incoherent mass of observations, which have been stored up with great care, but which, until they are connected by some presiding idea, will be utterly useless. The most effective way of turning them to account, would be to give more scope to the imagination, and incorporate the spirit of poetry with the spirit of science. By this means, our philosophers would double their resources, instead of working, as now, maimed, and with only half their nature. They fear the imagination, on account of its tendency to form hasty theories. But, surely, all our faculties are needed in the pursuit of truth, and we cannot be justified in discrediting any part of the human mind. And I can hardly doubt, that one of the reasons why we, in England, made such wonderful discoveries during the seventeenth century, was because that century was also the great age of English poetry. The two mightiest intellects our country has produced are Shakspeare and Newton; and that Shakspeare should have preceded Newton, was, I believe, no casual or unmeaning event. Shakspeare and the poets sowed the seed, which Newton and the philosophers reaped. Discarding the old scholastic and theological pursuits, they drew attention to nature, and thus became the real founders of all natural science. They did even more than this. They first impregnated the mind of England with bold and lofty conceptions. They taught the men of their generation to crave after the unseen. They taught them to pine for the ideal, and to rise above the visible world of sense. In this way, by cultivating the emotions, they opened one of the paths which lead to truth. The impetus which they communicated, survived their own day, and, like all great movements, was felt in every department of thought. But now it is gone; and, unless I am greatly mistaken, physical science is at present suffering from its absence. Since the seventeenth century, we have had no poet of the highest order, though Shelley, had he lived, would perhaps have become one. He had something of that burning passion, that sacred fire, which kindles the soul, as though it came fresh from the altar of the gods. But he was cut off in his early prime, when his splendid genius was still in its dawn. If we except his immature, though marvellous, efforts, we may assuredly say, that, for nearly two hundred years, England has produced no poetry which bears those unmistakable marks of inspiration which we find in Spenser, in Shakspeare, and in Milton. The result is, that we, separated by so long an interval from those great feeders of the imagination, who nurtured our ancestors, and being unable to enter fully into the feelings of poets, who wrote when nearly all opinions, and, therefore, nearly all forms of emotion, were very different to what they now are, cannot possibly sympathize with those immortal productions so closely as their contemporaries did. The noble English poetry of the sixteenth and seventeenth centuries is read more than ever, but it does not colour our thoughts; it does not shape our understandings, as it shaped the understandings of our forefathers. Between us and them is a chasm, which we cannot entirely bridge. We are so far removed from the associations amid which those poems were composed, that they do not flash upon us with that reality and distinctness of aim, which they would have done, had we lived when they were written. Their garb is strange, and belongs to another time. Not merely their dialect and their dress, but their very complexion and their inmost sentiments, tell of bygone days, of which we have no firm hold. There is, no doubt, a certain ornamental culture, which the most highly educated persons receive from the literature of the past, and by which they sometimes refine their taste, and sometimes enlarge their ideas. But the real culture of a great people, that which supplies each generation with its principal strength, consists of what is learnt from the generation immediately preceding. Though we are often unconscious of the process, we build nearly all our conceptions on the basis recognized by those who went just before us. Our closest contact is, not with our forefathers, but with our fathers. To them we are linked by a genuine affinity, which, being spontaneous, costs us no effort, and from which, indeed, we cannot escape. We inherit their notions, and modify them, just as they modified the notions of their predecessors. At each successive modification, something is lost and something is gained, until, at length, the original type almost disappears. Therefore it is, that ideas entertained several generations ago, bear about the same relation to us, as ideas preserved in a foreign literature. In both cases, the ideas may adorn our knowledge, but they are never so thoroughly incorporated with our minds, as to be the knowledge itself. The assimilation is incomplete, because the sympathy is incomplete. We have now no great poets; and our poverty in this respect is not compensated by the fact, that we once had them, and that we may, and do, read their works. The movement has gone by; the charm is broken; the bond of union, though not cancelled, is seriously weakened. Hence, our age, great as it is, and, in nearly all respects, greater than any the world has yet seen, has, notwithstanding its large and generous sentiments, its unexampled toleration, its love of liberty, and its profuse, and almost reckless, charity, a certain material, unimaginative, and unheroic character, which has made several observers tremble for the future. So far as I can understand our present condition, I do not participate in these fears, because I believe that the good we have already gained, is beyond all comparison greater than what we have lost. But that something has been lost, is unquestionable. We have lost much of that imagination, which, though, in practical life, it often misleads, is, in speculative life, one of the highest of all qualities, being suggestive as well as creative. Even practically, we should cherish it, because the commerce of the affections mainly depends on it. It is, however, declining; while, at the same time, the increasing refinement of society accustoms us more and more to suppress our emotions, lest they should be disagreeable to others. And as the play of the emotions is the chief study of the poet, we see, in this circumstance, another reason which makes it difficult to rival that great body of poetry which our ancestors possessed. Therefore, it is doubly incumbent on physical philosophers to cultivate the imagination. It is a duty they owe to their own pursuits, which would be enriched and invigorated by such an enlargement of their resources. It is also a duty which they owe to society in general; since they, whose intellectual influence is already greater than that of any other class, and whose authority is perceptibly on the increase, might have power enough to correct the most serious deficiency of the present age, and to make us some amends for our inability to produce such a splendid imaginative literature as that which our forefathers created, and in which the choicest spirits of the seventeenth century did, if I may so say, dwell and have their being.

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.

[769] Mr. Napier, in his _Memoirs of Leslie_, pp. 16, 17 (prefixed to
_Leslie's Treatises on Philosophy_, Edinb. 1838), says, that he
'composed the bulk of his celebrated work on Heat in the years
1801 and 1802;' but that, in 1793, he propounded 'some of its
theoretical opinions, as well as the germs of its discoveries.' It
appears, however, from his own statement, that he was making
experiments on heat, at all events, as early as 1791. See
_Leslie's Experimental Inquiry into the Nature and Propagation of
Heat_, London, 1804, p. 409.

[770] For specimens of some of his most indefensible speculations, see
_Leslie's Treatises on Philosophy_, pp. 38, 43.

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]

[771] Though he clearly distinguishes between the two. 'It is almost
superfluous to remark, that the term heat is of ambiguous import,
denoting either a certain sensation, or the external cause which
excites it.' _Leslie on Heat_, p. 137.

[772] 'Heat is an elastic fluid extremely subtle and active.' _Leslie on
Heat_, p. 150. At p. 31, 'calorific and frigorific fluid.' See
also pp. 143, 144; and the attempt to measure its elasticity, in
pp. 177, 178.

[773] 'Heat is only light in the state of combination.' _Leslie on
Heat_, p. 162. '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.'

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]

[774] 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 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.

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.

[775] '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.'

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]

[776] '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.

[777] '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.'

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]

[778] Mr. Napier, in his _Life of Leslie_, p. 17, says of it, very
gravely, 'Its hypotheses are not warranted by the sober maxims of
inductive logic.'

[779] '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.

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.