140 AIR AND LIFE. 



substance so nearly immaterial that it escapes our vision. However, 

 air is not always invisible; it may be seen very clearly; it may also be 

 touched and handled, although no one would undertake to do so, or to 

 recommend the feat. Whilst it is gaseous under normal pressure and 

 circumstances, it may be made to assume the liquid form when subjected 

 simultaneously to the influence of considerable cold and very high pres- 

 sure. High pressure alone is not sufficient. Under a pressure of 3,000 

 atmospheres, 3,000 times that of ordinary sea-level pressure, oxygen 

 and nitrogen remain gases (batterer) ; but if at the same time the 

 temperature is lowered, they immediately assume the liquid condition. 

 MM. Cailletet and Pictet have obtained liquid air by means of pressures 

 of 300, 500, or 1,000 atmospheres cooperating with intense cold, with the 

 cold corresponding to 100° or 200° below zero (Celsius or centigrade). 

 Under such circumstances air may even assume the solid form; the 

 liquid air freezes into a solid block. 1 



No one could venture to touch this liquid or solid with the bare skiu, 

 for two reasons; one being that, of course, air can be kept liquid or 

 solid only under the circumstances of its production, and instantly 

 becomes a gas under normal pressure or temperature; the other, that, 

 even if the transformation were not instantaneous, the intense absorp- 

 tion of heat (production of cold) which accompanies the passage from 

 the liquid or solid to the gaseous state would be more than sufficient 

 to kill instantly all living tissues in the vicinity. 



It is enough for our present" purpose to simply mention the impor- 

 tance of air as an elastic fluid, and the part played by this gas in 

 luminous, thermic, acoustic, and electric phenomena, where it is an all- 

 important medium. It is also sufficient to remind the reader of the 

 temperature of the atmosphere and its varied movements, from the 

 light breeze that cools the hot summer days to the cyclones and 

 tornadoes which destroy buildings and tear up the strongest giants 

 of the forests. Lastly, the atmosphere is very far from being unlimited. 

 It ceases at some distance from our planet, becoming very thin and rare 

 even at altitudes that are not exceedingly great, such as 5,000 meters 

 (Mont Blanc, 4,813 meters; Gaurisankar 8,810 meters), and while we 

 are not prepared to state the exact distance from the earth at which 

 all traces of air disappear, it is generally admitted that above 320 or 

 350 kilometers (1 kilometer =1,000 meters) height, vertically, there is 

 no atmosphere worth mentioning. Of course, at such altitudes, the air 

 must be exceedingly thin and rarefied, as it is in the exhausted receiver 

 of the air pump. 



It is commonly said that air is tasteless and odorless. Pure air 



1 The gas is first cooled down to 30° below zero, and then compressed under 200 or 

 300 atmospheres. It remains fluid; but if a small amount of it is then allowed to 

 escape, the sudden expansion — which is accompanied by a production of cold, while 

 compression causes heat to be evolved — cools down the remainder of the air, the 

 temperature falls to 200° below zero, and the air immediately assumes the liquid con- 

 dition. Lower down it freezes and becomes a solid block. 



