rTegulation of the heat loss. 



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It is obvious that the relative importance of these factors will vary 

 with conditions. Thus, high external temperatures will tend to 

 diminish the loss from radiation while increasing that from evapora- 

 tion, owing to the greater production and more rapid evaporation 

 of sweat. The variation in this respect is well illustrated by the 

 following table, compiled by Rubner, from experiments made upon 

 a starving dog:* 



It will be noted that between 25° and 30° C. there was a marked 

 increase in the loss of heat through evaporation. 



In man loss of heat is regulated chiefly by controlling the impor- 

 tant factors of evaporation and radiation. We accomplish this end in 

 part deliberately or voluntarily by the use of appropriate clothing. 

 Clothing of any kind captures a layer of warm and more or less 

 stationary air between it and the skin and in the interstices of 

 the fabric, and thus diminishes greatly the loss by radiation, since 

 air is relatively a poor conductor of heat. If the clothing is non- 

 porous and impermeable the captured layer of air next to the skin 

 becomes saturated with water vapor and thus retards the evapora- 

 tion of the sweat. 



The Hygiene of Clothing. — The hygienic value of clothing, from 

 this standpoint, rests mainly upon the way in which it influences 

 the radiation of heat from the skin, and upon the extent to which 

 it affects the rate of evaporation of sweat. These effects are de- 

 pendent in part upon the nature of the fabric, but in larger part 

 upon the mode of manufacture. The fabrics used for clothing are 

 made either from vegetable fibers (cotton and hnen) or from ani- 

 mal fibers (wool and silk), or from a combination of the two. The 

 animal fiber, especially wool, has the advantage of being less con- 

 ductive for heat and more hygroscopic, that is to say, has a greater 

 capacity for taking up water vapor. For these reasons it is well 

 adapted to protect the body from loss of heat and at the same 

 time to permit adequate ventilation of the layer of air next to 

 the skin, so that this layer is less liable to become saturated with 

 water vapor. In regard to the manufacture of these fabrics it is 



* Taken from Lusk, "Elements of the Science of Nutrition," Philadelphia, 

 1917. 



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