I70 BACTERIA, YEASTS, AND MOLDS 



are now on the market, differing considerably in detail but 

 alike in general principles. They all function through the 

 condensation and vaporization of a gas, the cooling being 

 effected by the drop in temperature produced upon vapor- 

 ization. The condensation is generally brought about 

 mechanically by means of an electrically driven pump, 

 although in one type the object is secured by heating the 

 gas at one point of the system by means of a flame and 

 cooling it at another by means of water. Whatever prin- 

 ciple is employed, the device enables the housekeeper to 

 obtain much lower temperatures than by means of ice. 

 Hence much more efficient refrigeration is available in the 

 household today than was possible in former years. 



In the mechanical refrigerator the temperature within 

 the freezing coil is below freezing. That place is, therefore, 

 not suitable for preserving many kinds of food; but is 

 rather used for freezing water and desserts. A frozen des- 

 sert may, however, be preserved almost indefinitely if 

 placed in a pan inside the freezing coil. Just below or just 

 beside the coil, a temperature as low as 35° is easily ob- 

 tained ; in fact an electric refrigerator may easily be set so 

 that freezing temperatures are secured just beneath it. 

 Accordingly the space either below or beside the coil (de- 

 pending on the model of refrigerator) is designed for storing 

 very perishable foods, such as milk and cream. Milk kept 

 at a few degrees above freezing practically never sours, and 

 it is always a surprise to the housewife who has had no 

 previous experience with such low temperatures to discover 

 how long it will remain sweet in this type of refrigerator. 

 Other undesirable changes may occur at this temperature, 

 however ; so it is hardly desirable to use, without cooking. 



