New Developments in Science 597 



which the thermostat is set, the current is again turned on, which 

 starts the controls that increase the amount of heat in the furnace. 

 Some thermostats have a clock, by which the heat may auto- 

 matically be turned off or on, or by which the temperature may 

 be changed, at any time required. 



Dry Ice. If you live in or near a large city, you may have 

 had delivered to you a package of ice cream that was so hard that 

 you could not cut it. On investigating you found in the ice-cream 

 package a small compact disk of flaky white material that looked 

 like compressed snow. This material was dry ice. 



Dry ice is formed by placing carbon dioxide under great 

 pressure and drawing off the heat that is generated by the pres- 

 sure. In the process, the gas becomes a white, flaky solid with a 

 temperature of about 109 degrees below zero. Dry ice gets its 

 name from the fact that it passes directly back into its gaseous 

 form instead of turning to a liquid as ordinary ice does. Dry ice 

 has several advantages over water ice. It never wets the food, and 

 requires no drainage pipes. One pound of dry ice is as effective 

 as fifteen or twenty pounds of water ice. For this reason dry ice 

 is coming into wide use for keeping materials cold during trans- 

 portation as it requires much less space than does water ice. The 

 space thus saved can be used to store more goods. When dry ice 

 is properly covered it evaporates very slowly. Meats and other 

 perishable goods, in freight cars and trucks, may thus be kept 

 cold with dry ice for several days without renewing the supply, 

 making long distance shipments possible. Dry ice is also used in 

 ice cream stores, meat markets, and other places where perishable 

 foods are kept. 



Air Conditioning. The country of Siam lies in southern 

 Asia, not far from the Equator. It is a hot country, with a moist, 

 unpleasant climate. Yet there is one place in Siam where people 

 are always comfortable in the palace of Siam's king and queen. 

 No matter how hot and steamy the air may be outside, the air 

 inside the palace is always cool and pleasant. Air conditioning is 

 the magic that makes this possible. 



The term air conditioning, or "indoor weather" as it is some- 

 times called, has come into use only within the last few years. It 



