quefying pressure at the temperature of the cooling
water, and the facility with which this pressure can be reached and
maintained, is of great importance in the practical working of any
refrigerating apparatus. Ammonia in its anhydrous form, the use of
which is specially dealt with in this paper, is a liquid having at
atmospheric pressure a latent heat of vaporization of 900, and a
boiling point at the same pressure of 371/2 deg. below zero F. Water being
unity, the specific gravity of the liquid at a temperature of 40 deg. F.
is 0.76, and the specific gravity of its vapor is 0.59, air being
unity. In the use of ammonia, two distinct systems are employed. So
far, however, as the mere evaporating or refrigerating part of the
process is concerned, it is the same in both. The object is to
evaporate the liquid anhydrous ammonia at such tension and in such
quantity as will produce the required cooling effect. The actual
tension under which this evaporation should be effected in any
particular case depends entirely upon the temperature at which the
acquirement of heat is to take place, or, in other words, on the
temperature of the material to be cooled. The higher the temperature,
the higher may be the evaporating pressure, and therefore the higher
is the density of the vapor, the greater the weight of liquid
evaporated in a given time, and the greater the amount of heat
abstracted. On the other hand, it must be remembered that, as in the
case of water, the lower the temperature of the evaporating liquid,
the higher is the heat of vaporization. It is in the method of
securing the rejection of heat during condensation of the vapor that
the two systems diverge, and it will be convenient to consider each of
these separately.

_The Absorption Process._--The principle employed in this process is
physical rather than mechanical. Ordinary ammonia liquor of commerce
of 0.880 specific gravity, which contains about 38 per cent. by weight
of pure ammonia and 62 per cent. of water, is introduced into a vessel
named the generator. This vessel is heated by means of steam
circulating through coils of iron piping, and a mixed vapor of ammonia
and water is driven off. This mixed vapor is then passed into a second
vessel, in order to be subjected to the cooling action of water. And
here, owing to the difference between the boiling points of water and
ammonia, fractional condensation takes place, the bulk of the water,
which condenses first,