CHAP. VI., 1.] 



HEAT. BLACK IRVINE. 



129 



the volumes or the weights of those ingredients were 

 equal. " Quicksilver, whether it were applied hot 

 to cold water or cold to hot water, never produced 

 more effect in heating or cooling an equal measure of 

 the water than would have been produced by water 

 equally hot or cold with the quicksilver, and only 

 two-thirds of its bulk." Of course, when equal 

 weights were used, the inequality of effect was still 

 more striking, for, from the great density of quick- 

 silver, it required no less than 30 times the weight of 

 the water mixed with it in order that it should con- 

 tribute in an equal degree to the production of a 

 mean temperature. Strange to say, the interpreta- 

 tion of this important experiment remained to be 

 made by Dr Black after half a century. He taught 

 that temperature is an effect of heat, which is neither 

 the same in all bodies nor in the same body under 

 differing circumstances ; that the superior effect of 

 the water to the mercury in determining the tempe- 

 rature of the mixture was caused by the fact, that it 

 is the nature of quicksilver to require a smaller 

 amount of heat to raise its temperature through 

 one degree than an equal volume or weight of 

 water would require under like conditions. Black 

 made many experiments to satisfy himself of the 

 constancy of this property in bodies ; and with the 

 assistance of Irvine, 1 probably ascertained its nume- 

 rical value (the amount of heat necessary to raise an 

 unit of mass of water through one degree of tempe- 

 rature being the standard) in different cases ; but he 

 left the subject chiefly in the hands of that observer 

 and of Watt. The former gave the name of ca- 

 pacity for heat to this property, which was after- 

 wards more happily termed specific heat by Gadolin, 

 who made many experiments on the subject, as did 

 also Cavendish and Adair Crawford whose theory 

 of animal heat, published in 1778, turns entirely on 

 this property of bodies. 



(589.) Irvine had the merit, such as it was, of proposing 

 " ne s f a theory on which his term capacity for heat was 

 apacity of principally founded, which occasioned for many years 

 odies for after, although now comparatively forgotten, much 

 discussion. He assumed that the changes in the 

 temperature of bodies, whether by alteration of 

 their mechanical condition or by chemical combina- 

 tion, were due to a change in the capacity for heat of 

 the substance or the mixture. He also assumed that 

 the total amount of heat contained in a body is pro- 

 portional to the amount of heat necessary to raise its 

 temperature through one degree (for example, a 

 pound of mercury contains altogether one-thirtieth 

 part of the entire heat contained in a pound of water). 

 From these principles he deduced the temperature of 

 absolute zero or privation of heat as follows : When 

 sulphuric acid and water are mixed together, the tem- 

 perature rises. This rise, according to him, is caused 

 by the capacity for heat of the mixture being less 



eat. 



than the average due to its ingredients. He has, there- 

 fore, got the ratio of the whole heat in the bodies 

 before mixture and after mixture. He has also got 

 the number of degrees of temperature corresponding 

 to this difference. Having the ratio of these quan- 

 tities, and also their difference, the quantities them- 

 selves, or the whole amount of heat expressed in de- 

 grees of temperature before and after mixture, become 

 known, and the temperature of absolute privation of 

 heat is also known. He applied the same reasoning, 

 with great ingenuity, to explain the latent heat of li- 

 quids and vapours, which he ascribed to their increasing 

 capacities. He thence deduced other values for the 

 absolute zero ; but whereas all these determinations 

 ought to have agreed, at least approximately, they 

 were found by later experimenters, especially by La- 

 voisier and Dal ton, to differ so widely even by se- 

 veral thousand degrees that, since the time of the 

 latter, this ingenious theory has been nearly aban- 

 doned, at least as far as the search after the zero is 

 concerned ; although undoubtedly change in the spe- 

 cific heat of bodies is often an important element in 

 determining their temperature. 



To return to Dr Black. From 1756 to 1766 he (590.) 

 filled the chair of Chemistry and Medicine at Glas- 



gow, where he also practised as a physician. In hischarac- 

 1766 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 engag- 

 ing 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, de- 

 scribed the whole delivery." 2 It is probable, also, 

 that in his private intercourse with his pupils, he in- 

 spired them with that love of research which distin- 

 guished his own early days, and his taste for neat and 

 accurate experiment could hardlyfail 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 inter- 

 ruption 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 



1 Sir J. Leslie has, I think inadvertently, given the credit of the discovery of specific heat to Dr Irvine. 2 Lord Brougham. 



