COMMUNICATION OF HEAT. 219 



non-conductors of heat. "\iVool, fur, hair and feathers, are also among the worst 



conductors of heat. 



"VToolens and furs are used for clothmer in cold weather, not 

 VThy are furs . ,,],,_ i 



and woolens because they impart any neat to the body, but because they 



used for cloth- ^^g ygj.™ ^y^^ conductors of heat ; and therefore prevent tho 

 ing? •' 



•warmth of the body from being di-avm otf by the cold air. 



The heat generated in the animal system by vitiJ action has constantly a 

 tendency to escape, and be dissipated at the surfoce of the body, and the rat© 

 at which it is dissipated depends on the difference between the temperatiu^ 

 o.' the surface of the body aud the temperature of the surrounding medium. 

 "By interposing, however, a non-conducting substance between the sm-faco of 

 the body and the external atmosphere, we prevent the loss of heat which 

 would otherwise take place to a greater or less degree. 



The non-conducting properties of fibrous and porous sub- 

 To what are gtances are due almost altogether to the air contained in their 

 the non-con- " j- j 



ducting prop- interstices, or between their fibers. These are so disposed as 



subsunce^^due? ^ receive and retain a large quantity of ah: without permittmg 

 it to circulate. 



The warmest clothing Ls that which fits the body rather loosely, because more 

 hot air -will be confined by a moderately loose garment than by one which fits 

 the body tightly. 



Blankets and warm woolen goods are always made with a nap or projec- 

 tion of fibers upon the outside, in order to take advantage of this principle. 

 The nap or fibers retain air among them, which, from its non-conducting 

 properties, serves to increase the warmth of the material. 



,^ . „ The finer the fibers of hair, or wool, the more closelv they 



What luflu- . , . , ,.,.., 1 ., • " 



ence has the retain the air enveloped within them, and the more imperme- 



fineness of the j^^^jg ^-jjgy ijecome to heat In accordance with this princi- 

 fibers upon the • . 



warnuli of a pie, the external coverings of animals vary not only with the 

 material ? cUmate which the species inhabit, but also in the same indi- 



vidual they change with the season. In warm climates the furs are generally 

 coarse and tliin, while in cold countries they are fine, close, hght, aud of uni- 

 form texture, almost perfect non-couductors of heat. 



We have illustrations of this principle also in the vegetable kingdom. The 

 bark of trees, instead of being compact and hard like the wood it envelops, 

 is porous and formed of fibers, or layers, which, by including more or less of 

 air between their surfaces, are rendered non-conductors, and prevent the 

 escape of heat from the body of the tree. 



An apartment is rendered much warmer for being furnished with double 

 d lors and windows, beaiuse tlie air confined between the two surfaces op- 

 poses both the escape of heat fi'om within, and the admission of cold from 

 vithout 



As a non-conducting substance prevents the escape of heat from within a 

 body, so it is equally efficacious in preventing the access of heat from without. 

 In an atmosphere hotter than our bodies, the cSect of clothing would be to 

 keep the body cooL Flannel is one of tho warmest articles of dresa, yet WQ 



