B L A 



476 



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tion, ami ho punued it experimentally with groat vigour 

 and commensurate succeM. 



The chemical subject which teems first peculiarly to have 

 excited his attention was connected with his profession as a 

 physician, and is thus detailed by Dr. Robison in the preface 

 to Dr. Black's ' Lectures on the Elements of Chemistry :' 



' It was the good fortune of chemical science that at this 

 time the opinions of professors were divided concern - 

 ing the manner in which certain lithontriptic medicines, 

 and particularly lime-water, acted in alleviating the excru- 

 ciating pains of the stone and gravel. The students usually 

 partake of such differences of opinion, and are thereby ani- 

 mated to more serious study, and science gains by their 

 emulation. This was a subject quite suited to the taste of 

 young Mr. Black, one of Dr. Cullcn's most zealous and in- 

 telligent chemical scholars. It was indeed a most interest- 

 ing subject, both to the chemist and the physician. All 

 the medicines which were then in vogue, as solvents of the 

 calculous concretion, resembled more or less the lapis in- 

 fernalis, and the common ley of the soap-boilers, two sub- 

 stances so terribly acrimonious, that in a very short time 

 they will reduce the firmest and most solid parts of the 

 animal body to a mere pulp. Therefore, while they were 

 powerful lithontriptics they were hazardous medicines, if in 

 unskilful hands. AH of them seem to derive their efficacy 

 from quick-lime, and this derives its power from the fire. 

 Its wonderful property of becoming intensely hot, and even 

 sometimes ignited, when moderately wetted with water, had 

 long engaged the attention of chemists. It was therefore 

 very natural for them to ascribe its power to igneous matter 

 imbibed from the fire, retained in the lime, and communi- 

 cated by it to alkalies and other substances, which it renders 

 go powerfully acrid. Hence undoubtedly arose the denomi- 

 nation of causticity, given to the quality so induced. I see 

 that Mr. Blank had entertained the opinion, that caustic 

 alkalies acquired igneous matter from quicklime. In one 

 memorandum he hints at some way of catching this matter 

 as it escapes from lime, while it becomes mild by exposure 

 to the air, but on the opposite blank page is written ' No- 

 thing escapes, the cup rises considerably by absorbing air.' 

 A few pages after this, he compares the loss of weight sus- 

 tained by nn ounce of chalk when calcined, with its loss 

 when dissolved by spirit of salt. Immediately after a me- 

 dical case is mentioned which I know to have occurred in 

 November, 1 752. From this it would appear that he had 

 before this time suspected the real nature of these sub- 

 stances. Ho had then prosecuted his inquiry with vigour : 

 the experiments with magnesia arc soon mentioned. 



These laid open the whole mystery, as appears by one 

 other memorandum : ' When I precipitate lime by a com- 

 mon alkali there is no effervescence : the. air quits the 

 alkali for the lime, but it is not lime any longer, hut c. c. c. 

 It now effervesces, which good lime will not.' He had now 

 discovered that the terrible acrimony of these powerful sub- 

 stances is their native property, and not any igneous pro- 

 perty derived from the lime, and by the lime from the fire. 

 He had discovered that a cubic inch of marble consisted of 

 about half its weight of pure lime, and as much air as would 

 fill a vessel holding six wine gallons, and that it was ren- 

 dered tasteless and mild by this addition, in the same 

 manner as oil of vitriol is rendered tasteless and mild in the 

 form of alabaster, by its combination with calcareous earth.' 



Having thus most satisfactorily proved to what the caus- 

 ticity of lime and the alkalies was owing, he made it the 

 subject of his inaugural thesis, which he entitled ' J)e Acido 

 a cibis orto, ct do Magnesia.' This occurred in 1754, when 

 the degree of doctor of medicine was conferred upon him by 

 the University of Edinburgh. In the following year he 

 published his ' Experiments on Magnesia, Quicklime, and 

 other Alkaline Substances.' In this the views which had 

 been hut little more than indicated in his thesis were de- 

 tailed at greater length, and the whole subject more fully 

 developed. 



Dr. Black's experiments and opinions respecting caus- 

 t.i considerable discussion; and they were 

 ;.illy attacked by Dr. Meyer of Osnnburg, who had 

 published a considerable volume on quicklime, in wliidi lie 

 professed to explain all the phenomena by the action of an 

 gue, formed in the lime during calcination, and 

 consisting of igneous matter in a certain inexplicable com- 

 bination with other substances. Though tln.-t work was 

 replete with injudicious experiments and incorrect reason- 

 ing, it gave Dr. Black considerable uneasiness , and without 



adding any fresh experiments, ho answered and refuted all 

 the objections which had been urged against him. 



In I 7iG, Dr. Cullcn having removed to Edinburgh, Dr. 

 Black was appointed professor of anatomy and lecturer on 

 chemistry in the University of Glasgow, where lie continued 

 till 1 766, when he was appointed to the chemical chair in 

 Edinburgh. Between the years 1759 and 1763 he matured 

 the speculations on heat which hail for a long period occa- 

 sionally occupied his thoughts. Bocrhaave has recorded nn 

 observation made by Fahrenheit, that water would become 

 considerably colder than melting snow, without freezing, 

 and would freeze in a moment if disturbed, and in the act 

 of freezing emitted many degrees of heat This notice seems 

 to have supplied Dr. Black with some vague notion that the 

 heat received by ice during its conversion into water was 

 not lost, but was contained in the water. The cxpern: 

 by which Dr. Black demonstrated the existence of what he 

 termed latent heat in bodies, are extremely simple and 

 of execution. He remarks ('Lectures,' vol. i. p. 119) that 

 ' melting ice receives heat very fast, but the only etV> 

 this heat is to change it into water, which is not in the least 

 sensibly warmer than the ice was before.' A great quantity 

 therefore of the heat, or of the matter of heat, which c. 

 into the melting ice, produces no other effect but to give it 

 lluidity, without augmenting its sensible heat; it appears to 

 be absorbed and concealed within the water, so as not to be 

 discoverable by the application of a thermometer.' 



' In order to understand,' he continues, ' this absorption 

 of heat into melting ice, and concealment of it in the v. 

 more distinctly, I made the following experiments: The 

 plan of the first was, to take a mass of ice, and an equal 

 quantity of water, in separate vessels of the same size and 

 shape, and as nearly as possible of the same heat, to sus- 

 pend them in the air of a warm room, and by observing 

 with a thermometer the celerity with which the wai 

 heated or receives heat, to learn the celerity with which it 

 enters the ice; and the time necessary for mcltinir the ice 

 being also attended to, to form an estimate from these two 

 data of the quantity of heat which enters into ice during 

 its liquefaction.' He exposed in the same room a | 

 quantity of water frozen into ice, and an equal quantity of 

 water at 33, and as the result of the experiment he stales. 

 ' that it was necessary that the glass with the ice receive 

 heat from the air of the room during twenty one half-hours, 

 in order to melt the ice into water, and to heat that water 

 to 40 of Fahrenheit During all this time it was receiving 

 the heat, or the matter of heat, with the same celerity (very 

 nearly) with which the water-glass received it during the 

 single half-hour in the first part of the experiment. For, 

 as the water received it with a celerity which was diminish- 

 ing gradually during that half-hour, in consequence of the 

 diminution of difference between its degrees of heat and 

 that of the air ; so the glass with the ice also received heat 

 with a diminishing celerity, which corresponded exactly 

 with that of the water-glass, only that the progression of 

 this diminution was much more slow, and corresponded to 

 the whole time which the water surrounding the ice re- 

 quired to become warmed to 40" of Fahrenheit. Tlie whi.V 

 quantity of heat therefore received by the ice-glass during 

 the twenty-one half-hours was twenty-one times the quan- 

 tity received by the water-glass during the single half-hour. 

 It was therefore a quantity of heat which, had it been 

 added to the liquid water, would have made it warmer by 

 (40-33)X21, or 7x21, or 147. No part of this heat how"- 

 ever appeared in the ice-water, except 8: the remaining 

 139, or 140 had been absorbed by the melting ice, and 

 were concealed in the water into which it was changed.' 



He then mentions that another obvious method of melting 

 ice occurred to him, in which it would be still more easy to 

 perceive the absorption and concealment of heat, by the 

 action of warm water. For the details of these very simple 

 yet most satisfactory experiments, we must content our- 

 selves with referring to Dr. Black's 'Lectures,' Tol.i. p. 123, 

 In page 157 of the same volume he proves that in the case 

 of boiling the heat absorbed docs not warm surrounding 

 bodies, but converts the water into vapour, and he adds, ' in 

 both ca-es, considered as the cause of warmth, we do not 

 perceive its presence: it is concealed, or latent, and I gave 

 it the name of latent heat.' It was indeed by Dr. Black's 

 doctrine respecting the nature of steam that Mr. Wall 

 led to his great improvement* in the steam-engine, a sulli- 

 cient proof, if indeed proof were required, of the immense 

 importance of his discoveries. 



