3. How should the light in a living-room be distributed? 4. What are the differences in direct, semi-direct and indirect lighting? 5. What is the purpose of a mantle for a gas or kerosene lamp? 6. What is the difference in burners to be used with and without mantles? 7. How is the light from a lamp measured? 8. Which lamp gives the greatest candle power of light for the amount of fuel used--the one with or the one without a mantle? PART IV COOLING DEVICES CHAPTER XVI REFRIGERATORS =116. Principles of Refrigeration.= Refrigerators (Fig. 56) are designed to prevent the rapid spoiling of food by keeping it too cool for the rapid growth of bacteria. They vary considerably in their efficiency, according to their construction and to the way in which they are managed. To preserve food and to save ice, the housewife must understand her refrigerator, and she must choose a good one. There is as much difference in the efficiency with which housewives manage their refrigerators as there are differences in refrigerators. [Illustration: FIG. 56. Refrigerator.] A series of experiments were conducted with a number of different makes of refrigerators. When the outside temperature was between 80 and 90 degrees Fahrenheit, and when the refrigerators were kept full of ice, it was found that the temperatures in different refrigerators varied between 45 and 60 degrees Fahrenheit. When the refrigerators were only partly full of ice, their temperatures rose several degrees. The refrigerators which held a temperature of 45 degrees when filled with ice, or with 100 pounds, used 25 pounds of ice each in three days, while in the same three days, the ones which could maintain only a temperature as low as 65 degrees, used 50 pounds each. The warmer the inside of a refrigerator, the faster the ice melts. In general, a refrigerator which maintains a low temperature is cheapest to operate. The refrigerator should be kept full of ice exposed so that it comes in contact with the air circulating within the refrigerator. The refrigerator which does not hold a low temperature will not only use more ice, but be less efficient in keeping food. =117. The Construction of Refrigerators.= The construction of a refrigerator should be such that it may be kept clean. There should be no cracks and corners to catch dirt and make breeding places for molds and bacteria. =118. Lining Refrigerators.= The best linings for refrigerators are porcelain, porcelain enamel, or glass for the more expensive ones, and galvanized iron or zinc for the less expensive ones. The shelves are usually made of heavy wire or of bent metal. The latter should be constructed so that they can be thoroly cleaned. [Illustration: FIG. 57. Diagram showing circulation in a refrigerator.]