196 THE HUMAN MECHANISM 



In what follows the student will learn that this is precisely 

 what the body is doing with regard to heat. We may, in- 

 deed, imagine a heat account of the body, the two sides of 

 which would be as follows: 



DEBIT CREDIT 



(Heat received) (Output of heat) 



1. Heat produced within the body. 1. Heat transferred to surround- 



2. Heat transferred to the body ing objects colder than the 



from warmer objects without body (by conduction, convec- 



(by conduction, convection, tion, and radiation). 



and radiation). 2. Heat lost in evaporating water 



of perspiration, etc. 



The balance of this heat account at any one time is the 

 amount of heat in the body, and this determines the temper- 

 ature of the body. When the output of heat exactly equals 

 the heat received, the balance of the account remains the 

 same; that is to say, the temperature is constant. A con- 

 stant temperature, therefore, means that the two sides of 

 the heat account are being kept equal to each other. If 

 the balance increases, either by the production of more heat 

 or by the loss of less, the temperature of the body rises, 

 and we have fever. 



11. Transfer of heat dependent upon the nature of the 

 vehicle of transfer. The rate at which heat may be trans- 

 ferred depends upon the nature of the substance through 

 which the transfer occurs and which we may speak of as the 

 vehicle of transfer. We cannot go minutely into the factors 

 here concerned, but would call attention to the following 

 points, which will be readily verified from experience: 



1. A gas is in general a poorer vehicle of heat transfer than 

 a liquid or a solid. We make use of this fact in the manu- 

 facture of fabrics for our warmer clothing, for these fabrics 

 are warm according to the quantity of air within their meshes. 

 A woolen garment is warmer than a cotton garment because 

 it contains within the fabric so large a quantity of the poorly 



