243 WELLS'S NATURAL rHILOSOPHT. 



circumstance will explain, in part, the reason of the very low temperatures 

 which exist at great elevations in the atmosphere. Persons ascending high 

 mountains, or in balloons, lind that the cold increases with the elevation. 

 The reason of this is, that as the air expands and becomes rarefied, its capac- 

 ity for heat is greatly increased, and it therefore absorbs its own sensibl© 

 heat. 



. In all quarters of the globe, the temperature of the air at a 



limit of per- certain height is reduced so low by its rarefaction, that water 

 peiualsnow? ^^^ ^^^ ^^^^ j^ ^ jj^^jj^j ^^^^^^ rj^j^jg jj^^^jf^ ^^^ height of 



■which varies, being the most elevated at the equator, and the most depressed 

 at the poles, is called the line of Perpetual Sxow.* 



Air forcibly expelled from the mouth feels cool ; in this instance the cold ia 

 due to a sudden expansion of the air, by which its capacity for heat is in- 

 creased. 



The capacity for heat also increases with the temperature. Thus it requires 

 a greater amount of heat to elevate the temperature of platinum from 212° to 

 213°, than from 32° to 33°. 



Of aU known bodies, water has the greatest capacity for heat. 



There are several diflerent ways by means of which the ca- 



HoTT may the pacitv of bodies for heat mav be determined. One method 



capacity for f J 



heat in differ- consists in inclosing equal weights of different bodies heated 



be ascertained? *° ^^^® ^^^^ temperature, in closed cavities in a block of ice, 

 and measuring the respective quantities of water which they 

 produce by melting the ice. 



The same result may also be obtained by what is called the method of mix- 

 tures. Thus, if we mix 1 pound of mercury at 66° with 1 pound of water at 

 32°, the common temperature will be 33°. Here the mercury loses 33° and 

 the water gains 1°; that is to say, the 33° of the mercury only elevates the 

 water 1°, therefore the capacity of water for heat is 33 times that of mercury ; 

 or, if we call the capacity or specific heat of water 1, then the capacity or 

 specific heat of mercury will be l-33d or .0303. 



In this way the capacities for heat of a great number of bodies has been 

 determined, and tables constructed in which they are recorded. In these 

 tables water is taken as the unit of comparison. 



All vapors are elastic, like air. 

 eiisticityofra- The tendency of vapors to expand is unlim- 

 *'°" ited ; that is to say, the smallest quantity of 



vapor will diffuse itself through every part of a vacant 

 space, be its size what it may, exercising a greater or less 

 degree of force against any obstacle which may have a 

 tendency to restrain it. 



• The line of perpetual snow at the equator occurs at a height of about 15,000 feet ; at 

 the Straits of Magellan, It occurs at an elevation of only 4,000 feet. 



