116 
JOUKNAL OF THE PLYMOUTH INSTITUTION. 
the increase of temperature is much less rapid than that of the 
pressure ; hence on escaping into the atmosphere the heat it 
contains expands it, and its temperature falls correspondingly. 
Further, solid snow and salt, each at 32°, on being mixed are 
liquefied by a portion of their own heat, and the temperature 
accordingly falls considerably below the freezing point of water. 
Lastly, though in most cases an increase of temperature is accom- 
panied by expansion, water in passing from 32° to 3 9°- 5 actually 
contracts. 
Dr. Black and his followers spoke of the heat which entered a 
body without raising its temperature as being Latent^ but as this 
is to confound temperature with heat, or to suppose the former to 
be the only effect of which the latter is capable, the term is now 
used as one of convenience only, and under protest. 
Though, all other things being the same, a change of temperature 
in the same lody is a trustworthy measure of heat, it fails to be so 
when different kinds of matter are operated on : thus, the heat 
which raises a given weight of water 1°, raises that of an equal 
weight of iron nearly 10°; the Specific heat of water exceeding 
that of iron in that ratio. 
Water at all temperatures is capable of giving off vapour 
( = steam = water + heat), and the temperatures of the two are equal, 
though they contain very different quantities of heat. As has been 
stated, steam at 212° has 990° of ''latent" heat. Steam at a lower 
temperature has an increase of -7° of '' latent" heat for each degree 
of diminished temperature: thus, the ''latent" heat of steam at 
182°, for example, is 990°+ (212 - 182)-7° = 990° + 21°= 1011°. 
That aqueous vapour should ascend through the air is a matter 
of necessity, since the specific gravity of the former is to that of 
the latter, the temperature being the same, as 5 : 8. Had it been 
heavier, however, it would have ascended in accordance with the 
law of the Interpenetration of Gases, which is, as experiments have 
shown, that aeriform bodies replace one another in volumes which 
are inversely as the square roots of their densities. 
The amount of aqueous vapour which a given space can contain, 
or which, in technical language, it requires in order to its saturation, 
is independent of the presence or absence of the atmosphere, and 
is a direct function of the temperature, as shown in the following 
table : — 
