In commercial practice the absorption system is very extensively employed. The ammonia is placed in a large cylinder known as a generator and is heated by steam coils. The ammonia vapor passes through an analyzer which traps any water vapor it may contain, and then it goes through a series of condensing coils, which are cooled by water. In these condensing coils the ammonia vapor is liquefied by the pressure in the generator and collects in a receiver, whence it passes through an expansion valve into the cooling coils in the brine tank. From the cooling coils the ammonia gas passes back through an absorber which reverses the operation of the generator. Here weak aqua ammonia is sprayed on the ammonia gas and absorbs it. The rich ammonia solution is then pumped into the generator. There are several auxiliary apparatus which are omitted in this brief description in the interest of clarity. THE COMPRESSION SYSTEM [Illustration: FIG. 82.--DIAGRAMMATIC VIEW OF A COMPRESSION MACHINE] One more type of refrigerating machine remains to be described and that is the compression type. This was invented by Jacob Perkins in 1834, but was not developed into a commercial machine until 1850. This machine is the most important of them all. In some respects it is like the absorption process, but in place of adding direct heat a compressing machine is employed. Figure 82 illustrates the system diagrammatically. At _A_ is the compressor which compresses ammonia gas. The gas which is heated by the compressor is then cooled and liquefied in the condenser _B_. Thence it passes into a brine tank _C_, where it expands and absorbs heat. From this point it is drawn back into the compressor, thus completing the cycle. In other words, the ammonia must absorb as much heat from the brine as is taken out of it at the condenser. CHAPTER XXIII ENGINES OF DESTRUCTION