498 A TEXTBOOK OF PHYSIOLOGY 



relatively a very slow process; but as aqueous vapour has a density 

 of only 0-62, air being taken as unity, and as the cooler air of the 

 atmosphere is warmed and expanded by the body heat, the body 

 itself sets up conventional currents which greatly accelerate the loss 

 of vapour. At 32 F. saturated air holds T ^ th of its weight of water 

 vapour, at 59 F. J^th, and at 86 F., 4 yh. The body warms up the 

 air in contact with it, and saturates it at skin temperature. The air 

 entangled in the clothes thus warmed and saturated cannot escape 

 easily if the atmosphere is still. Wind carries awaj* the air as fast 

 as, or much faster than, the skin warms and saturates it. Thus, 

 wind enormously increases the cooling of the body surface both by 

 convection and evaporation. The skin responds to a cold dry wind 

 by vaso-constriction and diminished transpiration of water. The 

 skin becomes pale and dry, and the heat loss is thus cut down. We 

 seek shelter, and put on more clothes. On warm, close days the skin 

 becomes flushed and moist, we throw off clothes, and make use of fans. 



Water vapour is a far better conductor than dry air, and thus 

 damp cold air feels raw and chill, while dry cold air is pleasant. Water 

 has 240 times the thermal conductivity and 3,000 times the heat capacity 

 of air. The particles of cold or even freezing water in a winter fog strike 

 the cold nerve-endings, and by cooling these give us an unpleasant chilly 

 sensation. The fog, by penetrating into our clothes, robs these of their 

 protecting value. The intensity of the temperature sense depends 

 on the difference between the blood-temperature within the cutaneous 

 capillaries and the surface temperature of the skin without. The fog 

 gives us a shower of cold particles of water, while a cold dry wind 

 constricts the bloodvessels of the skin, and, while having a far greater 

 cooling effect, dees not give us the same sensation of chill. 



Water vapcur, like glass, is almost opaque for the least refrangible 

 rays the infra-red and transparent for the middle luminous and 

 calorific radiations. Thus, on a cloudy day the water vapour both 

 scatters and absorbs the dark heat rays, and less heat reaches the earth. 

 On tre other hand, clouds after a sunny day, just as a glass-house, 

 prevent the escape of the dark heat rays from the earth, and cause 

 a warm night. On a clear night these rays radiate into space, and the 

 earth cools. The transparency and diathermancy of the air are 

 properties of the greatest importance, since living energy is derived 

 from the sun's light and heat. Water vapour and dust in the atmo- 

 sphere serve both to soften the scorching power of the sun and to 

 prevent the rapid scattering into space of the heat gained by the 

 earth. 



The motive-powers of the atmosphere are convection and evapora- 

 tion produced by the sun's heat . The winds arise from the displacement 

 of the warm moist and therefore lighter air by cold heavier air, and the 

 rain falls as the moist air becomes condensed in higher altitudes or 

 against cold land surfaces. The beauty of earth and sky, the glories 

 of sunrise and sunset, depend upon the particles of dust and vapour 

 in the' atmosphere. The particles reflect and scatter the shorter rays 

 and transmit the longer. At sunrise and sunset the oblique raj r s pass 



