water into vapour or steam; and he endeavoured to determine this quantity by experiment. He found that the latent heat in steam, which balanced the pressure of the atmosphere, was upwards of 800°. He also directed Dr. Irvine of Glasgow, one of his own pupils, to make an experiment for measuring the heat actually extricated from steam during its condensation in the refrigeratory of a still, which was found to be 774°. A few weeks after, Mr. James Watt made similar experiments on steam with a similar still; and the medium. result of these trials gave 825°.10 It may be observed that, in these 10 Ibid., pp. 144-174. I deem it of interest to give a few brief quotations from Dr. Black's lectures on the latent heat of steam, to indicate his views. "I immediately set about boiling off small quantities of water, and I found that it was accomplished in times very nearly proportional to the quantities, even although the fire was sensibly irregular. "My conjecture, when put into form, was to this purpose:-I imagined that during the boiling, heat is absorbed by the water, and enters into the composition of the vapour produced from it, in the same manner as it is absorbed by ice in melting, and enters into the composition of the produced water. And, as the ostensible effect of the heat, in this last case, consists not in warming the surrounding bodies, but in rendering the ice fluid, so in the case of boiling, the heat absorbed does not warm surrounding bodies, but converts the water into vapour. In both cases, considered as the cause of warmth, we do not perceive its presence it is concealed, or latent, and I give it the name of Latent Heat. "I put into a very strong phial about as much water as half filled it, and I corked it close. The phial was placed in a sand-pot, which was gradually heated. until the sand and the phial were several degrees above the common vaporific point of water. I was curious to know what would be the effect of suddenly removing the pressure of the air, which is well known to prevent water from boiling. The water boiled a very short while, but the ebullition gradually decreased, till it was almost insensible. Here the formation of more vapour was opposed by a very strong pressure proceeding from the quantity of vapour already accumulated and confined in the upper part of the phial, and from the increased elasticity of this vapour, by the increase of its heat. When matters were in this state, I drew the cork. Now, according to the common opinion of the formation of vapour by heat, it was to be expected that the whole of the water would suddenly assume the vaporous form, because it was all heated above the vaporific point. But I was beginning by this time to expect a different event, because I could not see whence the heat was to be supplied, which the water must contain when in the form of vapour. Accordingly, it happened as I expected: a portion only of the water was converted into vapour, which rushed out of the phial with a considerable explosion, carrying along with it some drops of water. But, what was most interesting to me in this experiment, was, that the heat of what remained was reduced in an instant to the ordinary boiling point. Here, therefore, it was evident that all that excess of heat which early experiments, the latent heat of steam was considerably underrated. Dr. Black also contributed to advance the knowledge of Specific Heat; but he chiefly left the development of this branch in the hands of his pupil Dr. Irvine, who was professor of chemistry in the University of Glasgow from 1769 to 1786, and to Mr. Watt, for both of whom he had the greatest respect. In 1766, Dr. Black was appointed professor of chemistry in the University of Ediuburgh, in succession to Dr. Cullen; and he filled. this chair with much credit to himself and advantage to the University, until his death in 1799. He was a very successful instructor; his lectures in the class-room were described by those who heard them as inimitable, and so interesting that they never failed to rivet attention.11 Thus his influence on the progress and the diffusion of science by his teaching for the long period of forty-three years, and his intercourse with society, was great and highly beneficial to his country and to the world. Another branch of the science of heat was taken up by Sir John the water had contained above the boiling point, was spent in converting only a portion of it into vapour. This is plainly inconsistent with the common opinion, that nothing more is necessary for water's existing in a vaporous form under the pressure of the atmosphere, than its being raised to a certain temperature. "This experiment was afterwards made by my friend Mr. Watt, in a very satisfactory manner. His studies for the improvement of the steam-engine gave him a great interest in everything relating to the production of steam.”—Pp. 159-160. In 1781, Dr. Black said to the students of his class:-"I think it sufficient to inform you that Mr. Watt, in the course of his studies on the steam-engine, has made all the necessary experiments with the most scrupulous care, knowing that the improvement of that noble engine must depend entirely on an exact knowledge of the procedure of nature in the formation and condensation of steam. Mr. Watt informs me that he has observed as exact coincidence between the heat rendered latent in the vapour, and that which emerges from it, as can be desired; and that the heat obtainable from steam, capable of sustaining the ordinary pressure of the atmosphere, is not less than 900° of Fahrenheit's scale, and that it does not exceed 950°."-Ibid., p. 174. 11 Professor Robson, one of his pupils, and the editor of his lectures, says that Dr. Black endeavoured every year to make his course of lectures more plain, and illustrated them by more examples in the way of experiment. So the students in his class "were not only instructed, but delighted; and he became a favourite lecturer, and many were induced, by the report of his students, to attend his courses." Leslie ; 12 he directed his attention to "Radiant Heat"-heat propagated from hot bodies to sensible distances. Sir John was educated at the University of St. Andrews, and early manifested a bent for mathematical studies. His work on the Nature and Propagation of Heat, which appeared in 1804, first brought him into notice; and the following year he was appointed to the chair of mathematics in the University of Edinburgh. The fact that heat is radiant, passing through space like light, was known at an early period; and various experiments had been made, and some of the phenomena which characterise it indicated; but heat in its radiant form was not systematically investigated till towards the end of the eighteenth century. The band of scientific men then engaged on this subject were Pictet, Prevost, Rumford, and Herschel; the first two were professors in Geneva, and were earlier in the field than Leslie. In 1791, Pictet's work entitled Essai sur le Feu appeared, which contains observations on latent and specific heat, and on the power of different surfaces to reflect and absorb it. He showed that radiant heat moves with great velocity. His treatise also embraced observations on hygrometry, on various points of meteorology, and on friction heat. He has the merit of establishing the meteorological observations at the convent of the Great St. Bernard, and thus commenced a series which has proved exceedingly interesting to scientific men. Prevost is the author of the theory termed the "Movable Equilibrium of Heat." His fundamental idea is that heat is a substance related with bodies of a highly elastic nature, continually given off from them in proportion to their temperature, which may represent the tension of the imaginary elastic fluid. Thus, when the temperature of a body is stationary, it is because it receives by radiation from surrounding bodies exactly as much heat as it parts. with in the same way. 13 His views were first published in 1791. Leslie was an ingenious and able experimenter. But unfortunately he started his researches with some rather dogmatic preconceptions; he had a notion that the pressure of air is essential to the propagation of heat; nevertheless, many of his experiments are interesting and valuable. He used a thermoscopic instrument constructed by himself, which he called the differential thermometer; it is an ingenious modification of the common air thermometer. He showed that the 12 Born in 1766, and died in 1832. 13 Dr. Forbes's Diss. Ency. Brit., p. 944. 1856. radiating or emissive effect of different surfaces varied from 100° to 12. He also showed by experiment that the radiation of heat from a plane surface proceeds with unequal force in different directions. When the specific heating power of the colorific rays is measured in a direction perpendicular to the surface whence it emanates, it is found to be at a maximum; and at any other angle with the surface, it varies as the sine of the angle. Afterwards this was also found to prevail in the case of light. His experiments to prove that the law of radiation of heat varies inversely as the square of the distance were not quite conclusive. He considered the influence of colour on the heating of bodies by original experiment; and it was found to be effectual only when the radiations were luminous. He engaged in long and ingenious researches touching the law of cooling bodies, embracing the effects of mass, surface, contact of air, currents of air, the cooling effects of different gases, and of air of different degrees of rarefaction. Besides his work on heat, his Dissertation on the Progress of Physical and Mathematical Science, and the articles on "Cold" and "Meteorology," in the seventh edition of the Encyclopædia Britannica, he is the author of Elements of Natural Philosophy (left unfinished), a Treatise on Geometry, and Philosophy of Arithmetic. He held the mathematical chair from 1805 to 1819, and in the latter year he was appointed to the chair of natural philosophy. He had a large and fine collection of apparatus, as indicated above, and devised many ingenious experiments. He was elected a corresponding member of the Institute of France in 1820; and, on the recommendation of Lord Brougham, he received the honour of knighthood in 1832. Since Leslie's time the science of heat has been greatly advanced; the dynamical theory of heat has been developed in the present century, and Scotsmen have contributed their share to the definite advancement of this branch of science. But it has been advanced to its present stage by a long list of scientific men. In 1812, Davy enounced that the direct cause of the phenomenon of heat is motion, and that the laws of its communication are precisely the same as the laws of the communication of motion. The researches into the radiation and absorption of heat mainly form the physical basis of Spectrum Analysis, which has greatly extended the power of ascertaining the constituent elements of the celestial bodies, the sun and the fixed stars. In the researches which ultimately led to these results, several Scotsmen have taken an honourable part. Professor Forbes discovered and demonstrated the polarisation of heat, and thus showed that radiant heat and light are the same. Among others who have contributed to advance Spectrum Analysis, I may mention Professor Stokes, Professor Balfour Stewart, and Sir William Thomson, of the University of Glasgow. Sir William Thomson (now Lord Kelvin) has taught the doctrine that there is sodium vapour in the sun's atmosphere, in his public lectures in the University of Glasgow, since the year 1852. Interesting conclusions touching the composition of the sun and of some of the stars have been reached :-"When we compare the spectra of different stars with that of the sun, we come to some very curious conclusions. We find four classes of spectra, as a rule, among the different fixed stars which have seemed of importance enough to be separately examined. The first class of spectra are those of white stars. You see an admirable example in Vega, and another in Sirius or the dog-star. All these white stars have the characteristic that they have an almost continuous spectrum with few dark lines crossing it, and these for the most part lines of hydrogen. These stars are in all probability at a considerably higher temperature than the sun. Then you come to the class of yellow stars, of which our sun is an example. In their spectra you have many more dark lines than in those of the white stars, but you have nothing of the nature of nebulous bands crossing the spectrum, such as you find in the third class; still less have you certain curious joins of shaded lines which you have in the fourth class of stars. This classification seems to point out the period of life, or phase of life, of each particular star or sun. When it is first formed, by the impact of enormous quantities of matter coming together by gravitation, you have very nearly continuous spectrum of a glowing whitehot liquid or solid body, or it may be dense gas, the sole, or nearly sole, absorbent being gaseous hydrogen in comparatively small quantity, and the spectrum having therefore few absorption lines. As it gradually cools, more and more of these gases surrounding its glowing surface become absorbent, and so you have a greater number and variety of lines. Then, as it still further cools, you have those nebulous bands which seem to indicate the presence of compound substances; which could not exist in the first two classes, because their temperature is so high as to produce dissociation. Still further |