orous form. It is, of course, well known that such a change can
only be brought about by the acquirement of heat; and for the purpose
of refrigeration (by which must be understood the abstraction of heat
at temperatures below the normal) it is obvious that, other things
being equal, that liquid is the best which has the highest heat of
vaporization, because with it the least quantity has to be dealt with
in order to produce a given result. In fact, however, liquids vary,
not only in the amount of heat required to vaporize them (this amount
also varying according to the temperature or pressure at which
vaporization occurs), but also in the conditions under which such
change can be effected. For instance, water has an extremely high
latent heat, but as its boiling point at atmospheric pressure is also
high, evaporation at such temperatures as would enable it to be used
for refrigerating purposes can only be effected under an almost
perfect vacuum. The boiling point of anhydrous ammonia, on the other
hand, is 371/2 deg. below zero F. at atmospheric pressure, and therefore for
all ordinary cooling purposes its evaporation can take place at
pressures considerably above that of our atmosphere. Some other agents
used for refrigerating purposes are methylic ether, Pictet's liquid,
sulphur dioxide, and ether. In this connection it should be stated
that Pictet's liquid is a compound of carbon dioxide and sulphur
dioxide, and is said to possess the property of having vapor tensions
not only much below those of pure carbon dioxide at equal
temperatures, but even below those of pure sulphur dioxide at
temperatures above 78 deg. F. The considerations, therefore, which chiefly
influence the selection of a liquid refrigerating agent are:

1. The amount of heat required to effect the change from the liquid to
the vaporous state, commonly called the latent heat of vaporization.

2. The temperatures and pressures at which such change can be
effected.

This latter attribute is of twofold importance; for, in order to avoid
the renewal of the agent, it is necessary to deprive it of the heat
acquired during vaporization, under such conditions as will cause it
to assume the liquid form, and thus become again available for
refrigeration. As this rejection of heat can only take place if the
temperature of the vapor is somewhat above that of the cooling body
which receives the heat, and which, for obvious reasons, is in all
cases water, the li