186 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



equal, because in the former the evaporation of sweat from the body and the 

 loss of \\at<r from the lungs are favored, the vaporization of water affecting 

 heat-dissipation more decidedly than the moisture of the air. Heat-dissipation 

 is more active in cold moist air than in cold dry air. Cold air is not favorable 

 to the vaporization of water, whereas cold moist air has a higher specific heat 

 than the dry air, and thus tends to carry oil" heat more rapidly. 



The character of the covering of the body is of great importance. This 

 is illustrated in the changes which occur in the natural covering of animals 

 during warm and cold seasons, and in the characters of the fur of species 

 which inhabit very cold or very warm climates. During the winter the fur 

 is longer and thicker than during the summer. Animals living in cold or hot 

 climates are supplied with a relatively greater or less abundance of fur or 

 feathers and subcutaneous fat. Man provides for changes of the seasons by 

 modifying the quantity and quality of his clothing. In the adaptation of 

 dress to climate, the conductivity, radiating coefficient, hygroscopic capacity, 

 porosity, weight, and color of the clothing are important factors. The poorest 

 conductors, other things being equal, make the warmest clothing; fur and 

 wool are poor conductors and therefore are adapted especially for cold seasons 

 and climates, while cotton and linen are good conductors and therefore make 

 cool clothing. The radiating coefficient depends upon the conductivity of the 

 material and the character of the radiating surface. The coarser the material 

 the better the radiating surface, hence the better the conductor and the cooler 

 the clothing. The hygroscopic character of the clothing is of far more 

 importance than is generally believed. Articles of clothing having a large 

 capacity for absorbing and retaining moisture are, other things being equal, 

 of more value, especially for underwear, than those possessing the opposite 

 quality. Woollen goods compared with those made of cotton not only have 

 ;i far greater absorptive capacity but retain moisture for a longer time. "When 

 the clothing is of wool people are less apt to catch cold from exposure to 

 draughts and sudden cold than when it is of linen or cotton, the wool pre- 

 venting a too rapid evaporation of moisture, thus guarding against chilling. 

 Porosity is a comparatively subsidiary factor. The greater the weight of the 

 clothing, other things being equal, the more is heat-dissipation hindered. The 

 color of the outer apparel has a certain influence owing to the relative heat- 

 absorbing capacities, black clothing being wanner than white, etc., hence the 

 genera] use of white or light-colored clothing in warm climates and seasons. 



A rise of internal temperature (bodily temperature) is favorable to an in- 

 crease of heat-dissipation, for several reasons: (1) I bat-production tends to 

 be increased and thus cause an effort of the system to get rid of the excess of 

 heat. (2) The activity of the circulation is increased, causing a larger amount 

 of blood to be brought to the cutaneous surface where it is subjected to the 

 influence of the cooler surroundings. (3) Respiratory movements are increased 

 so that heat-dissipation is favored by the larger amount of air respired and 

 larger amount of moisture carried off. (1) The temperature of the body is 

 higher in relation to that of the surroundings and thus heat-dissipation by 



