Opposite as it may seem, 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 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, part.i.tioning 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 part.i.tions 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.

More and more artificial ice is being used, even in localities where ice is formed naturally during parts of the year.

Many of the modern hotels are equipped with refrigerating plants where they make their own ice, cool their own storage-rooms, freeze the water in gla.s.s carafes for the use of their guests, and even cool the air that is circulated through the ventilating system in hot weather. In many large apartment-houses the refrigerators built in the various separate suites are kept at a freezing temperature by pipes leading to a refrigerating plant in the cellar. The convenience and neatness of this plan over the method of carrying dripping cakes from floor to floor in a dumb-waiter is evident.

Another use of refrigerating plants that is greatly appreciated is the making of artificial ice for skating-rinks. An artificial ice skating-rink is simply an ice machine on a grand scale--the ice being made in a great, thin, flat cake. Through the shallow tanks containing the fresh water coils of pipe through which flows the ammonia vapour or the cold brine are run from end to end or from side to side so that the whole bottom of the tank is gridironed with pipes, the water covering the pipes is speedily frozen, and a smooth surface formed. When the skaters cut up the surface it is flooded and frozen over again.

So efficient and common have refrigerating plants become that artificially cooled water is on tap in many public places in the great cities. Theatres are cooled during hot weather by a portion of the same machinery that supplies the heat in winter, and it is not improbable that every large establishment, private, or public, will in the near future have its own refrigerating plant.

Inventors are now at work on cold-air stoves that draw in warm air, extract the heat from it, and deliver it purified and cooled by many degrees.

Even the people of this generation, therefore, may expect to see their furnaces turned into cooling machines in summer. Then the ice-man will cease from troubling and the ice-cart be at rest.