so it needs to absorb the requisite amount of heat to
aid it in taking that form, and this heat it must take up from all
surrounding warm objects. It absorbs quickly all it can get out of
itself as liquid water, out of the glass vessel containing it, and from
the surrounding air. But the process of gasification with ebullition
goes on so quickly that the temperature of the water thus robbed of heat
quickly falls to 0 deg. C., and the remaining water freezes. Thus, then, by
pumping out the air from a vessel, _i.e._ working in a vacuum, we can
boil a liquid in such exhausted vessel far below its ordinary boiling
temperature in the open air. This fact is of the utmost industrial
importance. But touching this question of latent heat, you may ask me
for my proof that there is latent heat, and a large amount of it, in a
substance that feels perfectly cold. I have told you that a gasified
liquid, or a liquefied solid, or most of all a gasified solid, contains
such heat, and if reconverted into liquid and solid forms respectively,
that heat is evolved, or becomes sensible heat, and then it can be
decidedly felt and indicated by the thermometer. Take the case of a
liquid suddenly solidifying. The heat latent in that liquid, and
necessary to keep it a liquid, is no longer necessary and comes out, and
the substance appears to become hot. Quicklime is a cold, white, solid
substance, but there is a compound of water and lime--slaked lime--which
is also a solid powdery substance, called by the chemist, hydrate of
lime. The water used to slake the quicklime is a liquid, and it may be
ice-cold water, but to form hydrate of lime it must assume a solid form,
and hence can and does dispense with its heat of liquefaction in the
change of state. You all know how hot lime becomes on slaking with
water. Of course we have heat of chemical combination here as well as
evolution of latent heat. As another example, we may take a solution of
acetate of soda, so strong that it is just on the point of
crystallising. If it crystallises it solidifies, and the liquid
consequently gives up its latent heat of liquefaction. We will make it
crystallise, first connecting the tube containing it to another one
containing a coloured liquid and closed by a cork carrying a narrow tube
dipping into the coloured liquid. On crystallising, the solution gives
off heat, as is shown by the expansion of the air in the corked tube,
and the consequent forcing of the coloure