=e oe 
(589.) 
Irvine’s 
theory of 
Cuap. VL, § 1.] 
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,’ 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. 
Irvine had the merit, such as it was, of proposing 
a theory on which his term capacity for heat was 
capacity of Principally founded, which occasioned for many years 
bodies for after, although now comparatively forgotten, much 
heat. 
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 
HEAT.—BLACK—IRVINE. 
927 
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 Dalton, 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. 
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.”? 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. 
® Lord Brougham. 
: Tn his charac- 
1766 he left Glasgow for Edinburgh, to succeed Dr ter. 
