484 PHYSIOLOGICAL PHYSICS. [Chap, xxxix. 



perspiration, by conduction, and by radiation. How 

 great the loss by perspiration may be is readily under- 

 stood when one takes into account that the perspiration 

 passes off from the body in vapour, and that the trans- 

 formation into vapour means the abstraction from the 

 body of a large amount of heat which becomes latent in 

 the vapour. The tendency to increased temperature 

 of the body by increased external heat is counter- 

 balanced by increased afflux of blood to the skin, in- 

 volving increased perspiration, and therefore increased 

 abstraction of heat ; while external cold by its action 

 on the skin diminishes the supply of blood, and, in 

 consequence, the amount of perspiration, and so 

 diminishes the abstraction. In such ways a more or 

 less uniform temperature of 98'4 Fahr. (37 '6 C.) is 

 maintained by the human body. 



Loss of heat by the skin may be increased or 

 diminished, according as the clothing is a good or 

 bad conductor of heat. Reference to page 457 shows 

 how variously different substances used for clothing con- 

 duct heat, and how the hair and feathers of animals 

 are fitted to affect the loss of heat by conduction. 

 Besides the conductivity of clothes for heat, their 

 absorbing and emissive power determine their value 

 as warm or cold clothing. Rough clothing radiates 

 more readily than smooth. Colour does not seem to 

 affect the radiating power, contrary to the popular 

 opinion, as we have seen. Dark clothing, however, 

 absorbs heat most readily. The hygroscopic qualities 

 of clothing also determine its value, since if it 

 readily absorbs moisture from the skin, a great loss of 

 heat will be experienced. Finally, the compactness 

 of the ^cloth should be noted. The less compact the 

 material the more easily will the air penetrate it and 

 carry off heat by convection. 



