(584.) 
His disco- 
veries of 
latent and 
specific 
heat. 
Latent 
heat ab- 
sorbed in 
melting 
ice, 
Its amount. 
(585.) 
Conse- 
quences. 
926 
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, 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 
MATHEMATICAL AND PHYSICAL SCIENCE. 
[Diss, VI. 
occasion of every frost, and animal life would be im- 
possible. 
The analogy of the gradual formation of steam in 
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 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;? about two 
years later his pupil, Irvine, made experiments under 
(588.) 
boiling, to the gradual liquefaction of ice, was so Latent 
‘evident as to lead Black to conclude, without any het of 
vapour, 
Experi- 
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 W*- 
same inquiry soon after.? 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 (597) 
these discoveries of Black; it is the great interval Theae laws 
which separates the clear perception of a fact from singularly 
the explanation of it; or, in other words, from the apsrlookae 
before 
clear expression of the more general fact which em- plac. 
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 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 draw- 
ing such seemingly obvious conclusions. 
Heat, was in like manner a correct interpretation of Specific 
a fact already known. Boerhaave and Fahrenheit be**. 
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. 
In these early experiments the latent heat of steam was a good deal underrated. 
3 Ibid., 171-173. 
2 Black’s Lectures, i., 158. 
The other great discovery of Black, Specific (5g. : 
