other charge.

The plate system of artificial ice-making does away with the
discoloration and the cloudiness, because the water containing the
impurities or the air-bubbles is not frozen, but is drawn off and
discarded.

In the plate system, great permanent tanks six feet deep and eight to
twelve feet wide and of varying lengths are used. These tanks contain
the clean, fresh water that is to be frozen into great slabs of ice.
Into the tanks are sunk flat coils of pipe covered with smooth, metal
plates on either side, and it is through these pipes that the ammonia
vapour flows. The plates with the coils of pipe between them fit in the
tank transversely, partitioning it off into narrow cells six feet deep,
about twenty-two inches wide, and eight or ten feet long. In operation,
the ammonia vapour flows through the pipes, chilling the plates and
freezing the water so that a gradually thickening film of ice adheres to
each side of each set of plates. As the ice gets thicker the unfrozen
water between the slabs containing the impurities and air-bubbles gets
narrower. When the ice on the plates is eight or ten inches thick very
little of the unfrozen water remains between the great cakes, but it
contains practically all the impurities. When the ice on the plates is
thick enough, the ammonia vapour is turned off and steam forced through
the pipes so the cakes come off readily, or else plates, cakes, and all
are hoisted out of the tank and the ice melted off. The ice, clear and
perfect, is then sawed into convenient sizes and shipped to consumers or
stored for future use. Sometimes the plates or partitions are permanent,
and, with the coils of pipes between them, cold brine is circulated, but
in either case the two surfaces of ice do not come together, there being
always a film of water between.

Still another method produces ice by forcing the clean water in
extremely fine spray into a reservoir from which the air has been
exhausted--into a vacuum, in other words; the spray condenses in the
form of tiny particles of ice, which are attached to the walls of the
reservoir. The ice grows thicker as a carpet of snow increases, one
particle falling on and freezing to the others until the coating has
reached the required thickness, when it is loosened and cut up in cakes
of convenient size. The vacuum ice is of marble-like whiteness and
appearance, but is perfectly pure, and it is said to be quite as hard.

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