CHAPTER X 



WATER ITS RELATION TO METABOLISM AND THE 

 REGULATION OF BODY TEMPERATURE 



WATER vapour virtually forms one of the components of the atmos- 

 phere, and its presence is revealed by condensation as rain or cloud. 

 The quantity may vary from 1 to 32 grm. per c. metre of air. The 

 amount required to saturate the air varies with the temperature. 

 At 32 F. saturated air holds y^-g-th of its weight, at 59 F. g^th, and 

 at 86 F. ^th. For every 27 F. increase in temperature the 

 amount is doubled. The density of vapour is less than air as 

 0'623 is to 1*0. Thus a cubic foot of air saturated with water vapour 

 is lighter than a cubic foot of dry air. On this largely depends the 

 motive power of the atmosphere. The water vapour in the atmos- 

 phere exerts a most important influence on the heat of the sun, 

 and thus upon bioplasm. According to current hypothesis, the 

 material molecules of hot bodies put into vibration the contigu- 

 ous ether, and this transmits the vibratory movement as radiant 

 heat. In vacno the propagation of heat by radiation is perfectly 

 analogous to the propagation of light, and the velocity is the same. 

 The quantity of heat received by unit surface in unit time is inversely 

 as the square of the distance of the source of heat. In radiation 

 through matter part of the vibratory movement is transmitted to 

 the material molecules and absorbed. Dry air is almost equally 

 diathermic and transparent for all rays, and absorbs so little that 

 its presence can be neglected for small distances, and the rays re- 

 ceived by an object in it can be taken as equal to the inverse square 

 of the distance. Water vapour, 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 the other hand, clouds after a sunny day prevent 

 the escape of dark heat from the earth and cause a warm night. In a 

 clear night the dark heat radiates into space and the ground cools. 

 The transparency and diathermancy of the air are properties of the 

 greatest importance, since living energy is entirely derived from 

 sunlight and heat. Water vapour and dust serve both to soften 



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