he presence or absence of a quantity of
energy exhibited partly in the form of heat and partly in molecular
activity, which, for want of a better name, we are accustomed to call
"latent heat"; and to transform it from one state to another we have
only to supply or extract heat. For instance, if we take a quantity of
ice, say one pound, at absolute zero[11] and supply heat, the first
effect is to raise its temperature until it arrives at a point 492
Fahrenheit degrees above the starting point. Here it stops growing
warmer, though we keep on adding heat. It, however, changes from ice to
water, and when we have added sufficient heat to have made it, had it
remained ice, 283 degrees hotter or a temperature of 315 degrees
Fahrenheit's thermometer, it has all become water, at the same
temperature at which it commenced to change, namely, 492 degrees above
absolute zero, or 32 degrees by Fahrenheit's scale. Let us still
continue to add heat, and it will now grow warmer again, though at a
slower rate--that is, it now takes about double the quantity of heat to
raise the pound one degree that it did before--until it reaches a
temperature of 212 degrees Fahrenheit, or 672 degrees absolute (assuming
that we are at the level of the sea). Here we find another critical
point. However much more heat we may apply, the water, as water, at that
pressure, cannot be heated any hotter, but changes on the addition of
heat to steam; and it is not until we have added heat enough to have
raised the temperature of the water 966 degrees, or to 1,178 degrees by
Fahrenheit's thermometer (presuming for the moment that its specific
heat has not changed since it became water), that it has all become
steam, which steam, nevertheless, is at the temperature of 212 degrees,
at which the water began to change. Thus over four-fifths of the heat
which has been added to the water has disappeared, or become insensible
in the steam to any of our instruments.

It follows that if we could reduce steam at atmospheric pressure to
water, without loss of heat, the heat stored within it would cause the
water to be red hot; and if we could further change it to a solid, like
ice, without loss of heat, the solid would be white hot, or hotter than
melted steel--it being assumed, of course, that the specific heat of the
water and ice remain normal, or the same as they respectively are at the
freezing point.

After steam has been formed, a further addition of heat increases