art. (576.) liarly so. The action of light appears to be to reduce the metallic silver, at least partially, an operation which is completed in the daguerreotype process apparently by the affinity of the mercurial vapour for iodine and for oxygen.1 If anything were wanting to show the impossibility Importance of separating the scientific arts from the history of of the pho-science itself, it would be the case before us. The tographic art of photography is far in advance of the theory, yet it constitutes in itself the greater part of what we know in a highly interesting branch of Natural Philosophy the chemical agency of the spectrum. The surfaces of Talbot and Daguerre are the philosophical tools by which farther discoveries can alone be made. Stokes. (578.) Mr GEORGE GABRIEL STOKES, a fellow of Pembroke Professor College, Cambridge, and senior wrangler in 1841, at present holds the distinguished position of Lucasian Professor of Mathematics. It is almost need less, therefore, to state that his mathematical talents are generally acknowledged, and that he has displayed them by a ready application to many difficult problems in optics and mechanics which had not previously been accomplished. I may refer, however, to the integration of complex differential equations occurring in the theory of the flexure of solids (see art. 364), and in that of the rainbow (466), and in his elaborate investigation of certain cases of the friction of fluids (416). But it is more to our present purpose to observe that he combines this profound and technical command of analysis with singular skill in the experimental department of optics;-not merely in investigations closely connected with the wave theory, and expressible by mathematical formulæ, but in those which depend on the judicious questioning of nature by critical experiments not necessarily quantitative;-such, in short, as Newton discusses in his Optics; and indeed since Newton himself occupied the Lucasian chair, there have been perhaps few philosophers who have shown so remarkable an aptitude for both kinds of research. 2 (579.) In 1852 Professor Stokes announced that the reShows that frangibility of light, which has hitherto been consithe refran- dered its most inherent and invariable quality, may gibility of The fundamental in some circumstances be altered. light may be changed. experiment is this:-A beam of solar light is caused to form a pure and highly dispersed spectrum by Fluorespassing through two or three prisms in succession, cence. associated with a lens. The spectrum is made to fall on a glass vessel containing a solution of sulphate of quinine (a colourless fluid). Whilst the red, orange, and indeed the greater part of the luminous spectrum, pass through as if the fluid had been merely water, from about the middle of the violet "the path of the rays is marked within the fluid by a skyblue light, which emanates in all directions from the fluid, as if it (the medium) had been self-luminous." This appearance extends far beyond the visible violet, the presence of the invisible rays (called chemical) becoming disclosed by the reaction of the quinine; and the dark bands, both of the visible and the (usually) invisible spectrum, are marked by obscure planes traversing the mass of diffuse light. When the light thus emanating from the interior of the quinine is examined by a prism, it is found to consist of rays of various colours and refrangibilities corresponding to those of the ordinary spectrum; from which Mr Stokes concludes that, since only violet rays and the invisible rays beyond them could have furnished or excited this emanation, the rays of light themselves have been transformed into others, with a lower degree of refrangibility, and possessing the corresponding optical properties. In conformity with this explanation, Professor Stokes has considered and accounted for a number of curious phenomena described by Sir David Brewster and by Sir John Herschel; called by the former "internal dispersion," by the latter" epipolic dispersion," all of which, so far as they involve anything peculiar, may be reduced to the general principle that certain bodies by their action on light have the power of lowering the refrangibility of the rays incident upon them, whether belonging to the visible or invisible spectrum-that is, of emitting rays of a lower, while under the excitement of rays of a higher refrangibility. Hence the phenomenon has been termed the "degradation" of light. Mr Stokes has also called it "fluorescence," a word involving no hypothesis, being derived from the characteristic action of a certain kind of fluor-spar described by Sir David Brewster. Many substances, solid and fluid, are found to pos- (580.) sess these qualities. Amongst others, and in the Substances highest degree, glass coloured yellow by oxide of possessing uranium, called in commerce canary glass, and solu- perty. tions of horse-chestnut bark. this pro By using sources of light richer than sunlight in (581.) the highly refrangible rays; by also using quartz for Remarkthe prisms instead of glass (which has the power ment with able experiof absorbing these to a considerable extent)-Pro- electric light. 1 I may refer to a short but clear paper on the theory and practice of photography by the Rev. W. T. Kingsley, in the Journal of the Photographic Society, vol. i., August 1853. Mr Kingsley is one of the most scientific and practical improvers of the art. 2 This he has also done in a beautiful paper on the effect of polarization in modifying the phenomena of diffraction. Camb. Trans., vol. ix. 3 The greater part of these had been detected by Sir D. Brewster, with his usual acuteness and perseverance, in the course of his researches on "internal dispersion." The colouring matter of green leaves is one of the most remarkable. naming Mr Wheatstone's beautiful invention of the Stereoscope, as by far the most interesting contribution recently made to the theory of vision, regarded in a point of view not strictly anatomical. Although Mr Wheatstone's paper was published in the Philosophical Transactions for 1838, and the Stereoscope became at that time known to men of science, it by no means attracted, for a good many years, the attention which it deserves. It is only since it received a convenient alteration of form (due, I believe, to Sir David Brewster), by the substitution of lenses for mirrors, that it has become the popular instrument which we now see it, but it is not more suggestive than it always was of the wonderful adaptations of the sense of sight.] (582.) Heat considered in the 18th century as a branch of chemistry. CHAPTER VI. HEAT, INCLUDING SOME TOPICS OF CHEMICAL PHILOSOPHY. § 1. BLACK.-Latent and Specific Heat.-Irvine.-Hutton.-Doctrines of Heat applied to some Natural Phenomena. Down to the close of the 18th century, the science of Heat was studied and advanced mainly by chemists, and it was in all respects treated as a branch of Chemistry; a position of which we still find traces in the introduction of the doctrines of heat (even of radiant heat) into most of our approved treatises on Chemistry. This circumstance brings us, in this chapter, into close contact with the most illustrious chemical names of the second half of the last century and of the first years of the present. Such were Black, Cavendish, Lavoisier, and Dalton. Of the last and two first it may be doubted whether they were not as prominent discoverers in Physics as in Chemistry. Davy occupies a similar position. It was not, indeed, until the 19th century had made some progress that Chemistry assumed a strongly distinctive position of its own, and began to attain that large development and complex character of detail which render it a science now hardly accessible to those who do not devote to it their almost undivided attention. In the days of Black and Cavendish it was otherwise; and in the first section of the present chapter I shall attempt to give an outline of the characters of the very remarkable men who then advanced simultaneously the doctrines of Physics and Chemistry; referring, of course, chiefly to the former, but not entirely to the exclusion of the latter portion of their researches, particularly with respect to the atomic and gaseous theories of Dalton, which have a strongly physical aspect. I have elsewhere noticed the barrenness of the greater part of the 18th century in contributions to the experimental sciences; the temptation is therefore the greater to dwell a little even on the personal history of men so celebrated and influential as Black, Cavendish, and Dalton. as a che JOSEPH BLACK was born at or near Bordeaux in (583.) Black-bis France, in 1728. His biography, little eventful and eminence almost exclusively academic, has been recorded in some detail by his companions and friends Adam mist. Ferguson and John Robison (the former of whom was a relation), in the preface to the posthumous publication of his Lectures on Chemistry. It is sufficient for me to state that he entered the University of Glasgow as a student in 1746. Being destined for the medical profession, he removed in 1750 or 1751 to Edinburgh, where he benefited especially by the lectures of Cullen, a most eminent physician, and the author of a beautiful experiment on the cold produced during evaporation. Before Black graduated (in 1754) he had entered upon a course of chemical experiments connected with the causticity of many earthy bodies, which ended in his first (and perhaps most famous) discovery of the existence of fixed air or carbonic acid gas as an essential constituent of marble and other solids, together with a train of important consequences. Few inaugural dissertations have been so interesting to science as that on Magnesia, printed at Edinburgh in 1754, which contained these results. But on this purely chemical question we will not enlarge. 1 On Mr Stokes's experiments, see Phil. Trans., 1852-53; and Proceedings of the Royal Institution. veries of Latent heat absorbed in melting (584.) The discoveries of Black with which we are chiefly His disco- concerned are those of latent and specific heat. latent and The former, at least, is Black's sole and exclusive specific property. When we look back to the state of the heat. science of heat in the first half of the 18th century, we are surprised at the exceeding slowness of its progress. The thermometer had been improved by the use of the freezing and boiling temperatures of water for its fixed points, and the adoption of mercury in its construction by Fahrenheit; the correspondence of its scale with true increments of heat had been tested, though imperfectly, by Brooke Taylor; and Dr Martine of St Andrews had published an ingenious work (the best of its period) on the expansion of different liquids, and on some kindred subjects; but in general no great step was made until Black, in 1757, or previously, concluded that during the melting of snow or ice, a great quantity of heat enters into the body without affecting the thermometer in an appreciable degree. The heat therefore spends itself or is absorbed in effecting liquefaction. Black called it latent (as opposed to sensible) heat. He was led to this discovery by the very simple observation of the extreme slowness with which ice is melted by the application of an amount of heat which would have raised the temperature of water to an enormous extent; the thermometer plunged in the thawing ice remaining stationary until it is entirely reduced to water. When that occurs, heating immediately commences according to the usual laws. One of Black's original experiments clearly illustrates his mode of procedure. He suspended equal weights of ice at 32°, and of water as near as might be at the same temperature, in two thin glass vessels 18 inches apart, in a spacious room having a temperature of 47°. The vessel containing water rose 7 in temperature in half an hour, but the equal weight of ice had not wholly melted, nor had its temperature even slightly increased until 21 half hours had elapsed. Whence Black concluded (approximately) that as much heat is requisite merely to thaw ice as would raise an equal weight of water through 7 x 21 or Its amount. 147°; a result almost corrrect, although the experiment in this form is manifestly not unexceptionable. The converse process of freezing shows how prolonged must be the application of cold to discharge water at 32° of its latent heat, or heat of liquefaction, and to convert it into ice. ice. (585.) Consequences. Nothing is more admirable in these results than the light they throw upon certain natural processes. Were instant liquefaction the result of the smallest application of heat to snow at 32°, we should many times a year be the victims of uncontrollable floods; and did water instantly become ice on its temperature reaching the freezing point, our lakes and rivers would be rapidly consolidated to the very bottom on occasion of every frost, and animal life would be im- heat of vapour. The analogy of the gradual formation of steam in (586.) boiling, to the gradual liquefaction of ice, was so Latent evident as to lead Black to conclude, without any special experiment, that a great deal of heat becomes latent during the conversion of liquids into vapour. It appears to have been in 1762 that he attempted to determine roughly the amount, by a method similar to that just described for the heat of liquefaction. He compared the time required under the action of a uniform source of heat to raise the temperature of a certain quantity of water from 50° to the boiling point, with that required to boil it away; and inferring that heat was continually combining with the water at the same rate, he estimated the latent heat, or heat of vapour, to be as great as would have raised the temperature of the water had that been possible by 810°. This was in October 1762;2 about two years later his pupil, Irvine, made experiments under his direction with a still and refrigeratory, by which ments of he estimated the heat given out by steam during its Irvine and reconversion into water. Mr Watt prosecuted the same inquiry soon after. How happily Mr Watt applied the doctrine of latent heat thus brought experimentally under his notice, to the improvement of the steam-engine, has been already recorded. Watt. overlooked One thing strikes us very much on a review of (587.) these discoveries of Black; it is the great interval Theae lawg which separates the clear perception of a fact from singularly the explanation of it; or, in other words, from the before clear expression of the more general fact which em- Black braces it, although when once given, the explanation may seem to be almost expressed in the very enunciation of the fact. Nearly a century before Black, the lynx-eyed Hooke had noticed that water during the process of congelation remained unaltered in temperature, and that the same takes place when it boils, and this observation had been numberless times verified in ascertaining the fixed points of thermometers ; yet no one before Dr Black had even guessed that the heat which enters into ice during liquefaction, and into water during vaporization, remained as it were a constituent of the water and the steam thus formed, and could at any time be recovered by a converse process. Numberless instances occur in the history of science in which it requires the utmost attention to appreciate the importance and difficulty of drawing such seemingly obvious conclusions. The other great discovery of Black, Specific (588.) 1 On the authority of a letter from Black to Watt; see Black's Lectures. (589.) heat. the volumes or the weights of those ingredients were Irvine had the merit, such as it was, of proposing than the average due to its ingredients. He has, there- lecturer To return to Dr Black. From 1756 to 1766 he (590.) filled the chair of Chemistry and Medicine at Glas- Black as a gow, where he also practised as a physician. In his charac1766 he left Glasgow for Edinburgh, to succeed Dr ter. Cullen as Professor of Chemistry-a position which he held, with great credit to himself and with benefit to the University, till his death in 1799. His health, during the greater part of that time, was feeble, owing to a pulmonary affection, which often interrupted his lectures, and, it is stated, prevented him from engaging in severe study without immediate injury. Though he published one or two papers during these thirty years, they were of comparatively trifling importance. His influence on science was chiefly exerted through the medium of his pupils and of his intercourse with general society. His lectures are described by those who had the good fortune to hear them as inimitable of their kind-grave, dignified, and so interesting as to rivet the attention. "Perfect elegance as well as repose was the phrase by which every hearer and spectator naturally, and as if by common consent, described the whole delivery." It is probable, also, that in his private intercourse with his pupils, he inspired them with that love of research which distinguished his own early days, and his taste for neat and accurate experiment could hardly fail of being imparted to a certain extent. Yet we may be permitted to regret that his constitutional indolence, almost apathy, had perhaps as great a share as bad health in the interruption of his career of discovery. Even when quite a young man, his most interesting conclusions were so gradually evolved, that he himself had difficulty afterwards in fixing their date; and some of them were delayed even for years, until he found time to 2 Lord Brougham. 1 Sir J. Leslie has, I think inadvertently, given the credit of the discovery of specific heat to Dr Irvine. (591.) Dr Hutton -Black's intimate friend of rain and vapour. make the requisite experiments. So cool a temperament was not likely to grow warmer as age advanced. Almost as indifferent to the honours of discovery as his stoical contemporary Cavendish, unlike him he enjoyed in a high degree intercourse with the congenial society which Edinburgh at that time afforded. He loved to converse infinitely better than to write; especially when he could converse intimately with such men as Adam Smith, David Hume, Adam Ferguson, Principal Robertson, Dr John Robison, John Home, Clerk of Eldin, and Dr James Hutton. may HUTTON was perhaps Black's dearest friend, and be mentioned here (episodically) as no ordinary thinker in Natural Philosophy, as well as in Geology and Metaphysics. Besides the "theory of the Earth," his theory, which will ever bear his name, and which, after various transmutations in name and form, is now by far the most widely prevalent, his theory of Rain was an ingenious and important speculation. Other branches of Meteorology also claimed his attention, particularly, as might have been expected, those which are connected with the temperature of the earth. He was one of the first who drew conclusions from the temperature of springs, with regard to change of climate due either to increased latitude or to increased height above the sea. His hygrometer, in which the dampness of the air was estimated by the coolness due to evaporation, was unquestionably the first suggestion of a method now in general use. His ideas on the constitution of matter were bold and ingenious, though not on all points tenable. They resembled those of Boscovich, though independent of them. He published a voluminous treatise on several subjects in Natural Philosophy, and a still more formidable one on the Principles of Knowledge, neither of which attracted much attention at the time, and have been long forgotten; yet it is not unlikely that some of his speculations in metaphysics might be worth the labour of re (593.) Cavendish. and Hut examination. His friend and commentator, Playfair, (whose style was as remarkable for perspicuity as Dr Hutton's was the contrary) has drawn the following lively contrast between the characters of Hutton and Black, which may properly conclude this notice :— "Ardour and even enthusiasm in the pursuit of Contrast science, great rapidity of thought, and much anima- of Black tion distinguished Dr Hutton on all occasions. ton. Great caution in his reasonings, and a coolness of head which even approached to indifference, were characteristic of Dr Black. On attending to their conversation, and the way in which they treated any question of science and philosophy, one would say that Dr Black dreaded nothing so much as error, and that Dr Hutton dreaded nothing so much as ignorance; that the one was always afraid of going beyond the truth, and the other of not reaching it. The curiosity of the latter was by much the more easily awakened, and its impulse most powerful and imperiWith the former, it was a desire which he could suspend and lay asleep for a time; with the other, it was an appetite that might be satisfied for a moment, but was soon to be quickly renewed. Each had something to give which the other was in want of. Dr Black derived great amusement from the vivacity of his friend, the sallies of his wit, the glow and original turn of his expression; and that calmness and serenity of mind which, even in a man of genius, may border on languor and monotony, received a pleasing impulse by sympathy with more powerful emotions."1 ous. Black died on the 6th December 1799. His death, (592.) as recorded by his kinsman, Adam Ferguson,3 was Black's death. one of the most touching on record. It succeeded his customary state of health by an interval inappreciably short, and, as appeared by the accompanying circumstances, without the slightest physical emotion. The philosophic composure of his whole life was mirrored in the serenity of its close. § 2. CAVENDISH.4-His Singular Character and Attainments-Eminent Chemical DiscoveriesObservations on Heat and on other Branches of Physics-LAVOISIER-The CalorimeterTheory of Combustion and of Oxidation. Cuvier has justly remarked, in his biography of Cavendish, that he had to struggle in his scientific career against obstacles much more rarely encountered, and perhaps less easily overcome, than those which beset the progress of genius cramped by poverty and neglect. Cavendish was the descendant of one of England's noblest families, and he was likewise the possessor of enormous wealth; yet neither of 3 Preface to Black's Lectures, by Robison, p. lxxiv. 1 Playfair's Biographical Account of Dr James Hutton, Works, vol. iv. 2 Muirhead's Correspondence of James Watt. Introd., p. xxii. I find an apology almost necessary for introducing at some length the biography of Cavendish into a chapter professedly on Heat, his positive discoveries connected with which were less notable than in some other departments. But besides that his position in the first rank of chemists naturally indicates his place to be between Black and Dalton, I felt a wish to bring out the relief of the striking intellectual characteristics of those three remarkable men, by placing them in juxtaposition. I may add that these three sections were the earliest written of this Dissertation, at a time when I had hoped to interweave into its composition more of the purely biographical character of each period of scientific history than I found it afterwards practicable in all instances to carry out. I trust, however, that it may be found a not unwelcome variety amidst the abstruser details of science. In the case of Cavendish, too, so various are his claims on our notice, that it was inevitable to recur to them in different chapters, especially in those on Astronomy and Electricity. It was not, therefore, really material under which head the more strictly personal details were given to which I have alluded. |