In manufacturing low temperatures, by the Compression and Absorption systems, a liquifiable gas is used as the vehicle by which to impart cold to, and carry off heat from, the body to be cooled. Anhydrous-ammonia (ammonia from which all water has been removed) is usually employed. In the Compression machines, this gas is subjected to a pressure, averaging one hundred and fifty pounds per square inch, in a compression cylinder against a piston, which is operated by a steam engine. The heated gas is carried to a set of condenser coils, which are cooled by a water bath; here the gas is liquified by the reduction of temperature and the pressure. By this process it is made to part with the heat of compression, and its latent heat of vaporization, as well. The liquid ammonia is collected in a storage tank, and is then ready for refrigeration duty. From this point the Compression and Absorption systems are practically identical. The liquid ammonia is allowed to escape through a valve with a minute opening, into what are termed the expansion coils. As the ammonia enters it is freed from about three-fourths of the pressure at which it has been held, and begins to boil and vaporize. As heat is necessary to accomplish this, everything within reach of its influence is placed under tribute. As the gas parted with about five hundred and seventy heat units per pound at the condenser, its capacity for heat is now very large. The expansion coils may be placed in a loft of a cold storage room in the same position in which ice is placed for this duty. If they are submerged in a brine solution, the brine is cooled, and may be circulated in galvanized iron gutters suspended from the ceiling of the storage rooms. After circulating through the expansion coils, the gas is drawn out and forced again into the compression cylinder by a pump which renders the system a continuous one. IN MAKING ARTIFICIAL ICE, the expansion tubes may be submerged directly in the water which is to be frozen; the ice forms in huge cakes on the tubes, and is sawed into small cakes by a circular saw, when removed. This is termed the plate system. In the can plan, a large tank holds the expansion tubes, and, suspended from its upper side, are numerous iron cans; a brine solution completely fills the tank, and, being chilled by the tubes, it gradually freezes the water which has been placed in the cans for this purpose. IN THE ABSORPTION SYSTEM, aqua ammonia is placed in a retort containing a coil of steam pipes. A mixed vapor of water and ammonia is driven off, until sufficient pressure is developed in the retort to force the vapor through a small pipe into a condensing tank. Here the gas is cooled and liquified, and also rectified, or freed from water, making it anhydrous. The liquid passes into a receiving tank, and is then used for refrigerator work.