, heat is required to produce cold--for steam is
necessary to drive the compressor and pump of a refrigerating plant, and
fire of some sort is necessary to make steam.

The first artificial refrigerating machines produced cold by compressing
and expanding air, the compressed air containing the heat being cooled
by jets of cool water spirted into the cylinder containing it, then the
compressed air was released or expanded into a larger chamber and
thereby extracted the heat from brine or whatever substance surrounded
it.

It is in the making of ice, however, that refrigerating machinery
accomplishes its most surprising results. It was said in the writer's
hearing recently that natural ice costs about as much when it was
delivered at the docks or freight-yards of the large cities of the North
as the product of the ice-machine. Of course, the manufactured ice is
produced near the spot where it is consumed, and there is little loss
through melting while it is being stored or transported, as in the case
of the natural product.

There are two ways of making ice--or, rather, two methods using the same
principle.

In the can system, a series of galvanized-iron cans about three and a
half feet deep, eight inches wide, by two and a half feet long are
suspended or rested in great tanks of brine connecting with the
cooling-tank through which the pipes containing the ammonia vapour
circulates. The vapour draws the heat from the brine, and the brine,
which is kept moving constantly, in turn extracts the heat from the
distilled water in the cans. While this method produces ice quickly, it
is difficult to get ice of perfect clearness and purity, because the
water in the can freezes on the sides, gradually getting thicker,
retaining and concentrating in the centre any impurities that may be in
the water. The finished cake, therefore, almost always has a white or
cloudy appearance in the centre, and is frequently discolored.

In an ice-plant operated on the can system a great many blocks are
freezing at once--in fact, the whole floor of a great room is
honeycombed with trap-doors, a door for each can. The freezing is done
in rotation, so that one group of cans is being emptied of their blocks
of ice while others are still in process of congealing, while still
others are being filled with fresh water. When the freezing is complete,
jets of steam or quick immersion of the can in hot water releases the
cake and the can is ready for an