304 



LIQUEFIED AIR. 



careless handling. The most evident result of the 

 low temperature of the liquid is the dense fog that 

 surrounds it and proceeds from it, curling over the 

 sides of the containing vessel and falling to the 

 ground or floor where it gradually dissipates. In 

 draining off the liquid from the compressor this fog 

 acfiiinu lutes in a cloud or bank on the floor, and its 

 temperature is so low that the feet of the operator 

 may be frozen if he is not careful. 



The liquid is normally in a state of ebulli- 

 tion, whose violence varies with the surround- 

 ing temperature. On being poured, for instance, 



FREEZING RUBBER BALL. 



into an ordinary glass tumbler it begins by boil- 

 ing violently, but soon the glass and surrounding 

 air tocome cooled nearly to the temperature of 

 the liquefied air, the glass is densely coated with 

 hoarfrost, while the liquid becomes quieter and may 

 be seen to be of a steel-blue tint. A tumblerful of 

 it will boil away in a few minutes ; a larger quan- 

 tity 3 or 4 gallons in a double-walled can, packed 

 in felt, may last eight or ten hours. It must always 

 be kept and transported in an open receptacle 

 owing to the dangerously high pressure devel- 

 oped when it is confined ; hence to keep it for any 

 great length of time or to transport it to a distance 

 is at present an impossibility. Prof. Dewar has in- 

 vented a form of vessel known as the Dewar bulb, 

 which reduces the loss by evaporation to a mini- 

 mum, and in this about a gill of liquid air has been 

 kept with great care for about forty hours. The 

 I >i-war bulb is a double-walled glass bulb having the 

 air exhausted from the space between its walls and 

 having one wall silvered. The vacuum prevents 

 accession of heat by convection or conduction and 

 the polished metal shuts off radiation. 



Vaporization when the liquid is agitated or di- 

 vided is much hastened. When a cupful of it is 

 tossed into the air it descends in a shower of drops 

 like water, but most of these vanish as by magic 

 before they touch the ground. Objects placed in 

 the liquid arc quickly frozen solid. India rubber 

 becomes brittle and can be broken to bits with a 

 hummer; bread, eggs, or meat become similarly 

 brittle, and even paper can be crumbled in the hand 

 as if scon-lird. Sheet iron or tin plate can be 

 broken with the hand. Whisky and mercury are 

 frozen solid. The latter has the appearance o'f sil- 

 ver and a piece of it can be used to drive a nail or 

 to sustain a weight. If liquefied air be placed in a 

 teakettle over a fire the vapor is expelled through 

 the spout to a great height, and water poured into 

 the kettle is frozen solid while it is still over the fire. 

 The bottom of the kettle becomes covered with solid 

 carbon-dioxide in the form of "snow" while almost 

 close to the ral-hot coals of the fire. 



ProsHure. The boiling point of the liquid l.cin- 

 so low, the pressure necessary to bring this point up 

 to the ordinary atmospheric temperature that is, to 

 f-ause the evolution of vapor to cease is enormous. 

 If the liquid is confined, then-ton-, i-lmllition will 

 continue with the consequent production of gaseous 



air, until this enormous pressure is reached. In 

 practice the liquid can not be confined. Enough 

 pressure is developed in a small tube to project a 

 tightly driven cork with violence in a small fraction 

 of a second, and the pressure soon becomes danger- 

 ous. The liquid must be kept and transported in 

 open vessels, as noted above. 



Oxygenation. The boiling point of nitrogen 

 being 13 C. below that of oxygen, nitrogen is dis- 

 tilled out in the first boiling, so that the liquid be- 

 comes steadily richer in oxygen, whose proportion 

 very soon exceeds 75 per cent. The liquid as ordi- 

 narily experimented with is thus practically liquid 

 oxygen, and its vapor is gaseous oxygen. All the 

 familiar combustion experiments with oxygen can 

 be performed in the vessel in which it is boiling. 

 A sponge saturated with the liquid explodes when 

 lighted, paper soaked in it burns vividly, and a 

 steel pen or a watch spring burns when the end is 

 heated red-hot and dipped in it. If a rod of car- 

 bon, such as is used in the electric arc light be 

 heated to redness and then plunged into the liquid, 

 very vivid combustion takes place, yet, owing to the 

 low temperature of the liquid air, the other end of 

 the carbon may be held in the hand. In this ex- 

 periment the carbon at a temperature of about 

 5,000 F. and the air at 312 F. are in actual con- 

 tact, and this whole range of temperature may be 

 included within an ordinary glass tumbler. The 

 richness of the liquid in oxygen makes some of the 

 experiments with it dangerous. Thus the freezing 

 of whisky, alluded to above, produces a very pow- 

 erful explosive, and the bringing of a lighted match 

 into contact with the frozen alcohol would be dis- 

 astrous. 



The rapid increase in the proportion of oxygen 

 in the liquid is strikingly shown by pouring some 

 of it into water. Liquid nitrogen' is lighter than 

 water, having a specific gravity of 0.885, while liquid 

 oxygen is heavier than water, its specific gravity 

 being 1.124. The preponderance of nitrogen in the 

 mixture causes it to float at first, but, as the nitro- 

 gen boils away, drops of oxygen begin to sink into 

 the water. As they do so they are partially vapor- 

 ized, and the gaseous oxygen, tending to rise through 



EXPLODING A SPONGE. 



the water, communicates a peculiar whirling mo- 

 tion to the drop and drives it up again. This mo- 

 tion may be continuously repeated. The drops, 

 which may be as large as an inch in diameter, have 

 been called " oxygen divers." 



The magnetic properties of liquid oxygen may 

 be shown by filling a test-tube with it, allowing for 

 the escape of the vapor by a side tube. The test 



