the expansion coils in the freezing tank, and passes again through the cycle of operations just described. The same ammonia is thus used over indefinitely. The pressure to which the ammonia is subjected in this apparatus ranges from one hundred and twenty-five to one hundred and seventy-five pounds per square inch. The pump, shown in the lower part of Fig. 2, is one of several makes. It has two compression cylinders, seen at the top of the tall A-shaped frame. The piston-rods work vertically beneath these cylinders, and are connected by cranks and connecting-rods to the piston working in the steam-cylinder seen at the right. The use of the ammonia in making ice can be compared to the use of a sponge in baling a boat. As the sponge soaks up water from the bottom of the boat, and after being squeezed over the side is ready to soak up more, so the ammonia soaks up, as it were, heat from the water to be frozen; and, after this has been squeezed out by the compressor, the liquid is ready to take up more heat.
The water from which the ice is made in the New York factory, previously mentioned, is from the city supply (Croton). Before being frozen it is purified by filtering and distillation. It is first filtered, then converted into steam in vertical boilers about twenty feet high; the steam is condensed and again filtered in steam filters filled with coke. The condensation is effected by placing the filters in the open air on the roof of one of the buildings, and circulating around them water pumped from the river, near which the factory is located. After leaving the steam filters and condensers, the water is further cooled by passing through a cooler similar to the condenser used for the ammonia. After leaving the cooler, the water is filtered through charcoal, and is then ready to go into the cans. It is filled into them through a hose, which ends in a long nozzle, containing a patented device that prevents air from being carried down into the water. In order to make clear ice, the formation of air-bubbles in it must be prevented. Water always contains some air, which is driven out by boiling. When boiled water is frozen, the ice contains only what little air is absorbed by the water while it is being cooled down to the freezing-point. The artificial ice, therefore, is clear except a thin layer running lengthwise through the middle of the cake—the part that freezes last. A very attractive exhibit for a market is made by putting meat, fish, fruit, and flowers into cans of water and freezing them into the clear ice. Articles having smooth surfaces, and consequently few crevices in which air-bubbles can cling, give the best results.
It was mentioned early in this article that sulphurous oxide is used as a cooling agent in making ice. This is the choking gas that is formed when sulphur burns. An ice machine employing anhydrous sulphurous oxide is made, which, as it works accord-