542 THE AIR AND LIFE. 



dope lid in f>' on locality, time, ami many other conditions which I shall 

 not ennmerate, is of considerable importance with iei;ard to life. Air, 

 when too dry, iritates the respiratory organs ; when too moist it impedes 

 transpiration, or rather checks its beneficent effects. Va[)orized moist- 

 ure performs still another function of greater importance. It inter- 

 poses between the ground and the sky a beneficent screen which during 

 the day mitigates the sun's heat by absorbing a portion of the rays 

 and prevents it from scorching the ground and vegetation; and at 

 night, inversely, it precludes excessive cooling by radiation. In fine, 

 moisture allows the luminous rays of heat to jiass, but absorbs a grt'at 

 portion of the dark rays, v/hether they come from the sun, the earth, 

 or any other source. The experiments of Tyndall, and especially of 

 Pouillet, have jiroved that air, throngh the medium of its moisture, 

 absorbs nearly one-fourth of tlie solar heat, and allows only three- 

 fourths of it to reach the earth. If it were not for this screen, this 

 strainer, our summer days would be at the same time much warmer 

 and much colder, as is the temperature on high peaks or that which 

 is encountered at high altitudes in balloon ascensions. For the higher 

 the altitude the thinner the layer of vaporized water lying between 

 the snn and the observer, and under such circumstances the sun is 

 scorching. Its rays, meeting less obstruction, overheat everything, 

 and on the other hand the surrounding air is extremely cold, since it 

 contains but little moisture and absorbs very little heat. So that if 

 there were no moisture, our summer days would be torrid and at the 

 same time icy cold; we would be scorched by the sun, but the air 

 would remain cold, and in the shade very low temperatures would be 

 occasioned by the great radiation. 



At night moisture moderates radiation. The earth, heated during the 

 day, tends during the night to reliiupiish that heat and to return it to 

 the interplanetary spaces. This radiation is considerable when the 

 sky is very clear and the weather very dry; a clear night is colder 

 than a cloudy one; it is colder on summits having but a thin layer of 

 atmosphere and vaporized water above than in low lauds, over which 

 is spread a thicker atmospheric layer. Radiation is a phenomenon that 

 can not be avoided considering that the temperature i)revailing in 

 celestial si)ace is infinitely low, probably less than 100° below zero; but 

 it increases as the air becomes drier, and contains less moisture by 

 which the dark lieat rays emitted by the earth can be absorbed. In 

 the absence of moisture, the sun would no sooner set than considerable 

 refrigeration would take place, as it does on high mountains, on elevated 

 table lands, in Thibet for instance, or even in deserts like Sahara, where 

 the atmosphere is very dry; and such refrigeration would prove very 

 injurious and ev^en fatal to many animals and plants. Moisture, there- 

 fore, mitigates the heat of day and the cold of night ; it establishes some 

 uniformity where extremes unfavorable to life would otherwise alter- 

 nate. Let us also point to the ])artit tills in preventing total darkness 

 during the night by receiving light in altitudes which the solar rays 



