HUMIDITY OF AIR 445 



in great measure on the state of the superfieial blood vessels. 

 Dilatation of these enormously inereases the area. It is interesting 

 to note that the water content of the tissues varies with the age of 

 the person. Young and old {)e()ple have a higher water content 

 than adults in their prime (Chap. XXXV.). From age 20 to 50 

 man loses about 29 grams of water per hour in insensible perspira- 

 tion. Boys and old men lose more. 



(2) The effect of colour is merely a matter of the differential 

 absorption of radiant energy producing local heating in proportion 

 to the amount of energy absorbed and so causing a more or less 

 rapid evaporation. The black moist muzzle of the bull-dog is 

 much cooler than its white, comparatively dry skin. 



(3) The gradient of temperature has much to do with the 

 amount of heat lost by evaporation. This is an indirect effect. 

 It has been suggested that the pigmented skin of tropical races, 

 by absorbing radiant energy and so becoming unduly warm, stimu- 

 lates epidermal nerve-endings and produces vasodilatation and 

 profuse sweating. The evaporation of this water then cools the 

 body in a similar way to the Indian chatti or water cooler. Accord- 

 ing to G. F. Hearne, exhaustion of the sweat mechanism always 

 preceded heat-stroke in Mesopotamia. 



(4) The presence of a current of dry air removes the gaseous 

 water from the surface, and so brings dry air in contact with the 

 body. In other words, the partial pressure of the gaseous water 

 in the layer next to the body is kept at a minimum. This drying 

 effect of wind is operative irrespective of the temperature of the 

 wind, as witness the drying of shallow pools of water and of wet 

 clothes by cold and by warm breezes. 



(5) Humidity of Air. The presence of moisture in the air affects 

 its cooling powers in two ways, (a) Obviously, if the air is 

 already saturated with moisture it is unable to absorb more, and 

 evaporation from the body even when played upon by a draught 

 will be at a mininumi. {^) On the other hand, as we have already 

 mentioned, moist air is a much better conductor of heat than dry 

 air, and, therefore, the heat lost by conduction tends towards a 

 maximum in a humid atmosphere. The former of these factors 

 plays a major part when the surroundings are warm (over 70° F.), 

 the latter operates maximally in cold climates (imder Q5° F.). 

 This effect of humidity is clear when one considers the conditions 

 under which tobacco leaf is dried in Rhodesia. In the first stage 

 (drying at 40° C.) it is impossible for any one to enter the drying 

 sheds. The sheds are quite enterable during the second stage (at 

 almost 100° C). At this stage the leaf and, therefore, the air is 

 quite dry, while at 40° the leaf is fresh and giving oil moisture. 



