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 glass 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.