128 



(584.) 

 His disco- 

 veries of 

 latent and 

 specific 

 heat. 



MATHEMATICAL AND PHYSICAL SCIENCE. 



[Diss. VI. 



Latent 

 heat ab- 

 sorbed in 

 melting 



Its amount. 



(585.) 

 Conse- 

 quences. 



The discoveries of Black with which we are chiefly 

 concerned are those of latent and specific heat. 

 The former, at least, is Black's sole and exclusive 

 property. When we look back to the state of the 

 science of heat in the first half of the 18th century, 

 we are surprised at the exceeding slowness of its pro- 

 gress. 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 ingen- 

 ious 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, 1 concluded that during the melting of 

 snow or ice, a great quantity of heat enters into the 

 body without affecting the thermometer in an appre- 

 ciable 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 ex- 

 tent ; the thermometer plunged in the thawing ice 

 remaining stationary until it is entirely reduced to 

 water. When that occurs, heating immediately com- 

 mences 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 tem- 

 perature 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 

 147; a result almost corrrect, although the experi- 

 ment 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. 



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 tempera- 

 ture 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- 

 possible. 



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 heat 

 special experiment, that a great deal of heat becomes V 

 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 simi- 

 lar 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 in- 

 ferring 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 E xper i. 

 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 Watt - 

 same inquiry soon after. 3 How happily Mr Watt 

 applied the doctrine of latent heat thus brought ex- 

 perimentally under his notice, to the improvement 

 of the steam-engine, has been already recorded. 



One thing strikes us very much on a review of , 58 y \ 

 these discoveries of Black ; it is the great interval Theae law 

 which separates the clear perception of a fact from singularly 

 the explanation of it ; or, in other words, from the ^ oked 

 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 enun- 

 ciation of the fact. Nearly a century before Black, 

 the lynx-eyed Hooke had noticed that water during 

 the process of congelation remained unaltered in tem- 

 perature, and that the same takes place when it boils, 

 and this observation had been numberless times veri- 

 fied in ascertaining the fixe$ points of thermometers ; 

 yet no one before Dr Black had even guessed that 

 the heat which enters jnto 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 draw- 

 ing such seemingly obvious conclusions. 



The other great discovery of Black, Specific ($$8.) 

 Heat, was in like manner a correct interpretation of Specific 

 a fact already known. Boerhaave and Fahrenheit heat - 

 had found that when quicksilver and water were 

 mixed together, their temperatures being different, the 

 heat of the mixture was not, as might have been ex- 

 pected, the average of that of its ingredients, whether 



1 On the authority of a letter from Black to Watt ; see Black's Lectures. 2 Black's Lectures, i., 158. 



3 Ibid., 171-173. In these early experiments the latent heat of steam was a good deal underrated. 



