=Ammonia.=--Leaving for a moment the description of the apparatus to acquaint the reader with ammonia: In the form used in a compression refrigerating machine, it bears the name “liquid anhydrous ammonia” meaning in a “dry liquid” form differentiating from the common household ammonia which is a water solution containing ammonia gas in variable quantity. The distinction between these two is that “anhydrous ammonia” will evaporate to naught; from household ammonia the gas will evaporate but the water will remain. [Illustration: FIG. 15.--DIAGRAM SHOWING AMMONIA COMPRESSION SYSTEM.] Ammonia is a chemical compound made up of one part nitrogen and three parts hydrogen and is expressed by the chemical symbol NH₃. It has some peculiar characteristics being analogous to water. It will assume a solid, liquid and gaseous form, at -115° F. and -28¹⁄₂° F. for solid and liquid respectively and will modify to a gaseous form at any temperature above -28¹⁄₂° F. under “atmospheric” or conditions of no pressure. Water, as is known, changes from a liquid to a solid at 32° F. and changes to a vaporous form at 212° F. but here the analogy ceases. Ammonia has further peculiar advantages for use in refrigerating production that it becomes a liquid at variable pressures and temperatures; for example, when the temperature is reduced to 60° F. under a pressure of 92 pounds, when reduced to 80° under a pressure of 139 pounds and at 100°, under a pressure of 200 pounds; with variables above, between and below these conditions. Ammonia, as will be seen, has variable forms and capacities under the conditions imposed upon it. Substances passing from a liquid to a gaseous form require heat to make this change and consequently absorb it; the complement to the giving up of heat when the process is reversed and the substance changed from a gaseous to a liquid form. The adoption or harnessing of these principles is the nucleus upon which mechanical refrigerating effect is built. A pound of water passing to steam will absorb about one thousand degrees of heat, ammonia has the same characteristic to a different degree. They both return the heat when the process is reversed. =Type of Machine Illustrated.=--The diagram illustrates a single acting pump showing its piston, a liquifier or condenser where the ammonia is modified in form from a gas as received therein to a liquid; a receiver to which it flows and a cooler in which it is expanded and where the heat brought to it by the returning brine is picked up by the ammonia and carried to the condenser. The arrows indicate the direction of flow.