No substance will change from a liquid to a gaseous form unless heat be supplied to perform the work of making this change. At the outset it was stated ammonia will boil or evaporate at 28¹⁄₂° below zero Fahr. when under no pressure or in the open air. Further, if ammonia be contained in a vessel and the pressure be reduced below atmosphere, the boiling point is lowered still further. For example at 10.6 vacuum gauge pressure the boiling point will be 40° below zero. Imagine a brine cooler such as a tank with coil submerged and surrounded by a brine solution: This brine is circulated through the building and by common knowledge we know it absorbs heat and is returned warmer than it was sent out. It is supplying the heat for boiling the ammonia gas. Reverting to the expansion valves: Assume there is a pressure of 180 pounds in the condenser and a liquid temperature of perhaps 85 to 90 degrees; the same conditions existing in the receiver; also assume a pressure of 15 pounds on the ammonia coils which will produce a temperature condition of zero. In the diagram, the coil in the tank is attached to the expansion valve while on the other end of the coil is made the connection to the pump or compressor. This by its action is withdrawing the ammonia gas as rapidly as it is generated, due to the tightness of the piston, which if it is properly fitted will pump a vacuum upwards of 22 to 25 inches upon the system attached to it, unless gas be supplied to fill the space. In operation, the expansion valve is opened slightly, the liquid is freed into a space where the pressure is lowered, a condition created for expanding the ammonia to a gas and the heat contributed by the surrounding brine is absorbed by the ammonia changing from a liquid to a gaseous form. =The Cycle.=--The cycle, therefore, is a gas in the expansion tank at low pressure and temperature, admitted to the pump or compressor under this condition, compressed to a small volume and increased in pressure, discharged in this condition to the condenser where it becomes a liquid and in a condensed form at a lowered temperature and ready to return to the expansion tank to be used over again. Ammonia gas is the heat carrier. If it be used in packing house service, either it absorbs the heat directly from the rooms in which the animals are suspended or the brine in the tank is circulated through the rooms to absorb the heat and carry it back to the cooler or expansion tank.