The essential parts of the refrigerating system employing ammonia is represented in Fig. 157. The _compressor_ exhausts ammonia gas from the coiled pipe in "_E_" and compresses the gas in "_C_," where under 150 pounds pressure and the cooling effect of water it condenses to liquid ammonia. This is allowed to pass slowly through the regulating valve, whereupon it evaporates and expands in the long coiled pipe in "_E_" on its way back to the compressor. This evaporation and expansion causes a large amount of heat to be absorbed from the brine, cooling the latter below the freezing point of pure water and thus permitting the freezing of cans of water suspended in the brine. The chilled brine may also be sent through pipes in order to cool storage rooms containing meat or other food products. The ammonia absorbs heat when it vaporizes and gives up heat when it is compressed and liquified. Important Topics 1. Heat of vaporization, of water 536 calories per gram. 2. Boiling point, effect of pressure upon boiling point, laws of boiling. 3. Distillation, artificial cooling, freezing mixtures, refrigeration by evaporation. Exercises 1. How much heat is required (a) to melt 1 g. of ice at 0°C., (b) to raise the temperature of the water resulting to 100°C., (c) to change this water to steam? 2. If the water leaving a steam radiator is as hot as the steam how is the room warmed? 3. What is the effect of placing salt upon icy sidewalks in cold weather? 4. Is rain water distilled water? Is it perfectly pure? 5. What are two advantages of the high heat of vaporization of water? 6. If the heat from 1 g. of steam at 100°C. in changing to water and cooling to 0°C. could be used in melting ice at 0°C. how much ice would be melted? (3) HEAT AND WORK =191. Necessity for Heat Energy.=--From early times man has been able to transform motion into heat, and has used this ability in many directions as in starting fires and warming himself by friction. It took man many centuries, however, to devise an effective machine for transforming heat into mechanical energy or to use it in doing work. The _power_ of a man is small and as long as the work of the world had to be done by man power, progress was retarded. When man began the use of beasts of burden, he took a long step in advance since one man could then employ and direct the power of many men in the animals he controlled. Man also built water-wheels and windmills thus gaining power directly from the forces of nature and these added much to his working ability. But he took the greatest step in gaining control over his surroundings when he learned to use heat energy and to make it drive his machines.