HEAT. 
that the colimn of mercury ‘in these three 
cases, was equal to 0.2, 3.5, and $0 inches respectively. 
y making a few experiments, and observing the ratio 
aN the numbers in these cases bore to each other, 
_ he constructed a table containing every degree of heat, 
_ from 0° on Fahrenheit’s scale to 825°: (Manch, Mem. vol. 
v. p. 559.) Experiments of a similar kind have been per- 
formed in France by M. Bettancour. As the boiling 
int of liquids is lowered or raised, according as the 
erical pressure upon them is diminished or in- 
creased, it has found, that if the pressure be aug- 
‘ in a very , as is the case when 
great de, 
water is inclosed in Papin’s digester, it is capable of re- 
ceiving*a much higher temperature, than what it can 
maintain under ether circumstances. Water has, in this 
» been heated 200° or 300° beyond its usual boiling 
point ; and it has then acquired the power of dissolving 
many substances, over which it has no action in its or- 
dinary state. 
-Itis upon the property which liquids possess, of being 
convertible into the us state, that the processes o 
evaporation and distillation depend. The first of these 
is performed, when our object is to separate the more 
volatile parts of a compound, for the purpose of procu- 
ring those that are less so; and the second, on the con- 
trary, is done in order to procure the volatile part itself. 
A minute account of these operations belongs more to 
chemistry, and to the various branches of chemical ma- 
_mufactures, than to the general view which we are ta- 
- king of the effects of heat. (See Distimuavtion and Eva- 
PorATION.) In both cases, the separation is effected b 
the combination of heat with the liquid ; inthe one, it is 
sometimes produced by the heat which naturally exists 
inthe atmosphere, aided by a current of air, continually 
presenting a new stratum to be acted upon; and at other 
times a quantity of heat is thrown into the substance by 
the combustion of some kind of fuel. It is, however, ne- 
cessary, in either case, that a large surface should be ex- 
ge to the atmosphere, for the purpose of carrying off 
vapour as rapidly as it is produced; for it appears 
that it is principally at the surface that it is generated. 
In the process of distillation, artificial heat is always em- 
ployed,and commonly ina greater degree than where the 
object is merely to produce evaporation. Here the liquid 
is confined in close vessels; and the operation depends 
upon a portion of it, which is nearest the source of heat, 
becoming converted into vapour, and passing up through 
the other parts of the fluid in the form of bubbles, con- 
stituting what is styled ebullition. 
' We have already remarked, that when a solid is con- 
verted into a liquid, or a liquid into an elastic fluid, the 
f conversion is brought about suddenly. The substance 
in question, before it changes its state, continues to re- 
ceive heat, is expanded in a certain degree, and has its 
temperature raised; but if an additional quantity of 
heat be still given to it, the expansion no longer goes 
on in the same manner, and the temperature is no 
longer elevated, but it assumes a new form, becoming, 
‘according to circumstances, either a liquid or a vapour. 
It was formerly supposed, that this change did not de- 
pend upon any peculiar or specific action, but that the 
mere addition of a certain small portion of heat was 
adequate to effect it. Dr Black, however, perceived the 
insufficiency of the opinion usually entertained upon 
the subject, and was induced to investigate it with 
great assiduity ; the result of which was, the establish- 
ment of his celebrated theory of latent heat. The fun- 
damental position of this theory is, that when a solid is 
where it does not exist in a sensible stote. See page 677.—E>. 
683 
converted into a tani oo 9h id into a gas, a much 
greater 4 wre of heat is a by it than is per- 
ble y the sensations, or the thermometer, the ef- 
fect of which is to unite with the particles of the body, 
and thus to alter its form. When, on the contrary, the 
vapour is reduced to the state of a liquid, or a liquid to 
that of a solid, heat is disengaged from it, without the 
substance in question indicating any diminution of tem- 
perature, either to the sensations, or to the thermome- 
ter. This phenomenon is considered to be the reverse 
of the former ; here the heat that escapes is not sup- 
posed to have affected the body in the way that free 
caloric acts, but simply to have maintained it in the 
state of elastic fluidity, or of liquidity, 
Effects af 
Heat. 
_—e 
We are hence led to regard heat as existing in two pig 
states that are essentially different from each other: the sy 
one producing temperature and ex 
sion, and of course free and 
capable of being measured by the thermometer; the '**t bt 
other not manifesting these properties, but uniting it~ 
self to bodies, and changing their form. The former 
f we call free or uncombined heat; the latter latent or 
combined heat. As we are in the habit of conceiving 
temperature and expansion * to be the most certain in- 
dications of the presence of heat, it required a train of 
minute experiments to establish a position which ap- 
peared, at first view, so much at variance with our or- 
dinary conceptions, as that heat could be communi, 
cated to bodies without producing these effects. The 
following facts may be considered as the ground-work 
of Black's doctrine. Leta mass of pounded ice, cooled 
several di below the freezing point, be exposed to 
the heat of a furnace, so that it may receive equal quan- 
tities of caloric in equal intervals of time. ‘The ice, as 
it receives the heat, becomes warmer at each succes- 
sive interval, until the whole acquires the temperature 
of 32°; but now, although the heat still remains ap. 
lied as before, the ice acquires no additional warmth.; 
It still indicates 32° to the thermometer, but it is ob- 
served gradually to dissolve. No increase of “at pe 
ture takes place until the whole is rendered fluid, and 
then it again begins to grow warmer, and as before ac- 
uires successive increments of heat, until it arrives at 
the 212th degree. Here, again, the process ceases ; and 
whatever quantity of heat be sent into the fluid, its 
temperature remains unchanged ; but now the fluid is 
observed gradually to assume the state of vapour, until 
the whole of it is converted into the aeriform state, 
Now there is every reason to conclude, that in this case 
the ice and water continue to receive equal quantities 
of heat for the whole period ; yet during some part of 
the process, the thermometer remains stationary. At 
this very time, however, the change of form occurs ; 
and therefore it is natural to conclude, that it is con. 
nected with the absorption of a portion of heat, which, 
in consequence of its not ucing either tem 
or expansion, has obtained the name of latent. The le- 
gitimate inference is, that a certain quantity of caloric, 
which would otherwise have been employed in 
cing temperature and expansion, is necessary in the one 
case for melting the ice, and in the other for vaporizing 
the water. 
What we have now stated may be regarded as the 
Proofs of 
the theory 
of latent 
heat. 
Deductions 
foundation of the celebrated theory of latent heat ; from the 
which has been universally received, as affording a sa~ ‘eory- 
tisfactory explanation of some of the most interesting 
phenomena that occur in the system of nature. A 
number of yery important consequences were immedi- 
ately deduced from it, of which we may enumerate the 
* It will be seen, in another part of this article, that expansions and contractions are produced by heat in glass and other substances 
