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 another 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.