262 



HEAT. 



The light and porous nature of snow renders it eminent- 

 ly serviceable as a clothing to the earth in the depth of 

 winter, preventing the escape of the heat from below, and 

 protecting the roots of plants from injury or destruction. 

 Hence the very severity of the cold of the Northern re- 

 gions, by producing an abundance of those beautiful 

 feathery crystals which form snow, becomes the means of 

 protecting from its own effects the tender herbage buried 

 beneath this amj^le shelter. 



CONDUCTING POWER OP LIQUIDS. 



Fij. 23G 



Liquids are found to conduct heat very slowly, and 

 they were for a long time considered perfect non-conduct- 

 ors. Some interesting experiments have been performed 

 in illustration of this property. A large glass jar may be 

 filled with water (fig, 286), in which may be fixed an 

 air thermometer, which is always quickly sensitive to 

 small quantities of heat. A shallow cup of ether, floating 

 -pi 287. i^^^^' above the bulb, may be set on 

 fire, and will continue to burn for 

 some time before any effect can be 

 seen upon the thermometer. The 

 upper surface of a vessel of water 

 lias been made to boil a long time 

 with a piece of unmelted ice at the 

 bottom. Liquids are found, how- 

 ever, to possess a conducting power 

 in a very slight degree. 



When a vessel of water is heated 

 in the ordinary way over a fire, 

 tlie heat is carried tiirough it merely by the motion 

 of its particles. The lower portion becomes warm,' 

 and expands; it immediately rises to the surface, and 

 colder portions sink down and take its place, to ascend in 

 their turn. In this way, a constant circnlation is kept up 



