becomes thicker and thicker, until it is finally a solid mass
of clear crystal ice, usually with a small core of opaque or snowy ice,
exactly through the centre.

As fast as their contents are frozen the cans are removed by a special
lifting apparatus, and dipped for a minute into hot water to loosen the
block from the can. Then it slides out easily, and is stored away for
use.

There are other factories conducted on a somewhat different plan from
the foregoing, in which the ice is made to form on iron plates, in cakes
weighing several tons each.

In such factories the brine-chamber is in the shape of double partition
walls of iron plates, about four inches apart. The partition divides a
deep wooden water-tank into two equal rooms, and in the narrow space
between the iron plates the brine and pipes for the ammonia gas are
placed. The rooms are filled with pure water, which is in contact with
the brine-chamber on one side. Ice soon begins to form on the iron side
plates, precisely in the same way as on a pond or river, except that the
sheet of ice is vertical instead of horizontal. Only about half of the
water in the rooms is allowed to freeze.

[Illustration: A BLOCK OF MANUFACTURED ICE.]

When the cakes of ice are considered to be of sufficient thickness, the
cold brine is pumped out of its compartment into another tank, and its
place is filled with water of ordinary temperature. This soon thaws the
ice cakes loose from the plates, and allows the mass of ice to be lifted
out by hoisting machinery. The ice is then passed on to the
sawing-machine, which divides it into blocks weighing about two hundred
pounds each. The only essential difference in the two systems described
lies in the fact that in the can method all the water is frozen, and if
there be any impurity in the water the ice will contain it. In the plate
method the ice is formed entirely from one side of the cake, and only
about one-half of the water is allowed to congeal into solid ice. Since
water, in freezing, tends to purify itself in the way in which the
natural ice of ponds and rivers purifies itself, the plate method more
nearly resembles the natural way, and the ice shows its characteristic
structure.

After having performed its work in cooling the brine, the expanded gas
is drawn from the pipes by means of powerful steam-pumps, and it is then
compressed into a coil of iron pipes kept immersed in a tank of cold
running water. This compression of