COMMUNICATION OF HEAT. 221 



has not free access into them ; in consequence of which they remain almost at 



an even temperature, wliich in summer is about 10 degrees colder, and in 



winter about 10 degrees warmer than the external air. 



„ , 500. Refrigerators, used for the preservation of animal and 



Upon what , , , . , , , , ,, , 



principle are vegetable substances in warm weather, are double-walled 



refrigerators boxes, with the spaces between the sides filled with powdered 

 and fire-proof ^ c i 



safes con- charcoal, or some other porous, non-conducting substance. 

 Btructed? rpj^^ so-called "fire-proof" safes are also constructed of 



double or treble walls of iron, with intervening spaces between them filled 

 with gjT^sum, or " Plaster of Paris." This lining, which is a most perfect 

 non-conductor, prevents the heat from passing from the exterior of the safe 

 to the books and papers within. The idea of applying " Plaster of Paris" in 

 this way for the construction of safes, originated, in the first instance, from a 

 workman attempting to heat water in a tin basin, the bottom and sides of 

 which were thinly coated with this substance. The non-conducting proper- 

 ties of the plaster were so great as to almost entirely intercept the passage of 

 the heat, and the man, to his surprise, found that the water, although directly 

 over the fire, did not get hot. 



501. It has been already stated that liquids and gases 

 are non-conductors of heat, and can not well be heated, 

 like a mass of metal, or any solid, by the communication 

 of heat from particle to particle. 



Why can not This pcculiarity is owing to the mobility 

 gHses'^be hMN whicli subslsts among the particles of all fluids, 

 mannerls's™!! ^^^ ^0 ^^^ chaugc iu the slzc of the particles, 

 "*^ which is invariably produced by a change in 



their temperature. 



The constituent particles of solid bodies being incapable of changing their 

 relative position and arrangement, the heat can only pass through them, from 

 particle to particle, by a slow process ; but when the particles forming any 

 stratum of liquid are heated, their mass, expanding, becomes 

 lighter, bulk for bulk, than the oolder stratum immediately 

 above it, and ascends, allowing the superior strata to descend. 



How is water 502. When the heat enters at 

 made hot? ^|^g bottom of a vesscl containing 

 water, a double set of currents is immediately 

 established — one of hot particles rising to- 

 ward the surface, and the other of colder par- 

 ticles descending to the bottom. The por- 

 tion of liquid which receives heat from below 

 is thus continually diffused through the other 



