56 



AIR AND FIRE 



FIG. 49. 



A Dewar Bulb 

 for Holding 

 Liquid Air. 



57. Liquid Air. In the liquid form air is colorless, 

 and has about the same density as water. It is composed 

 of liquid oxygen and nitrogen. Liquid air is constantly 



evaporating; to prevent explosions we keep 

 it in open vessels called "Dewar bulbs," 

 from the inventor (Fig. 49). They have 

 double walls, and the air is removed from the 

 space between the walls, so that heat from 

 the outside cannot affect the liquid within. 

 The walls of the bulbs are silvered for the 

 same reason. "Thermos" and "Caloris" 

 bottles are made on the principle of Dewar 

 bulbs, to keep a cold liquid from getting warm and a hot 

 liquid from getting cool (cf. Ex. 12, 68). Air is made 

 liquid at a low temperature by great pressure. It boils 

 at about -190 C. (Fig. 50). 



Alcohol held in liquid air becomes solid, like ice. Mercury is 

 changed to a hard metal, and may be used as a hammer head, to drive 

 nails. A rubber ball put into liquid air be- 

 comes brittle, and flies into fragments when 

 thrown on the floor. A piece of meat be- 

 haves in the same way. Liquid air that 

 actually wets the skin burns it like white- 

 hot iron; yet the hand may be put into the 

 liquid for a moment without injury, because 

 a layer of gaseous air covers the hand like a 



glove. FIG. 50. 



Liquid Air Boiling 

 on Ice. 



58. How the Atmosphere is Puri- 

 fied. Since the air receives impurities from the breathing 

 of animals and plants, from all burning, and from all 

 decay, why does it not become foul, and unfit to breathe? 



