the
temperature again at a much faster ratio to the quantity of heat added,
which ratio also varies according as we maintain a constant pressure or
a constant volume; and I am not aware that any other critical point
exists where this will cease to be the fact until we arrive at that very
high temperature, known as the point of dissociation, at which it
becomes resolved into its original gases.

The heat which has been absorbed by one pound of water to convert it
into a pound of steam at atmospheric pressure is sufficient to have
melted 3 pounds of steel or 13 pounds of gold. This has been transformed
into something besides heat; stored up to reappear as heat when the
process is reversed. That condition is what we are pleased to call
latent heat, and in it resides mainly the ability of the steam to do
work.

[Graph: Temperature in Fahrenheit Degrees (from Absolute Zero)
against Quantity of Heat in British Thermal Units]

The diagram shows graphically the relation of heat to temperature, the
horizontal scale being quantity of heat in British thermal units, and
the vertical temperature in Fahrenheit degrees, both reckoned from
absolute zero and by the usual scale. The dotted lines for ice and water
show the temperature which would have been obtained if the conditions
had not changed. The lines marked "gold" and "steel" show the relation
to heat and temperature and the melting points of these metals. All the
inclined lines would be slightly curved if attention had been paid to
the changing specific heat, but the curvature would be small. It is
worth noting that, with one or two exceptions, the curves of all
substances lie between the vertical and that for water. That is to say,
that water has a greater capacity for heat than all other substances
except two, hydrogen and bromine.

In order to generate steam, then, only two steps are required: 1st,
procure the heat, and 2nd, transfer it to the water. Now, you have it
laid down as an axiom that when a body has been transferred or
transformed from one place or state into another, the same work has been
done and the same energy expended, whatever may have been the
intermediate steps or conditions, or whatever the apparatus. Therefore,
when a given quantity of water at a given temperature has been made into
steam at a given temperature, a certain definite work has been done, and
a certain amount of energy expended, from whatever the heat may have
been obtained, or whatever b