98 EFFECT OF HEAT ON OASES. 



mercury 12.64 inches high, together with the atmospheric 

 pressure, which by the barometer is shown to be equal to a 

 column of 29.5 inches ; hence the elastic force 



= (12.64 + 29.5) inches = 42.14 inches. 



2. If the level in the closed branch rise 6.4 inches, find 

 the height to which the level in the open branch should 

 rise, the barometer standing at 30.42 inches, and DE being 

 10 inches. Ans. 60.48 inches. 



49. Effect of Heat on Gases. When a given quan- 

 tity of air or gas is increased in temperature, it is found 

 that, if the air or gas cannot change its volume, its elastic 

 force is increased ; but if the air can expand freely, while 

 its elastic force remains the same, its volume will be in- 

 creased. 



To illustrate this, take an air-tight piston in a vertical 

 cylinder containing air, and let it be in equilibrium, the 

 weight of the piston being supported by the cushion of air 

 beneath it. Raise the temperature of the air in the cylinder 

 by immersing it in hot water; (1) the piston will rise in the 

 cylinder as the volume of the heated air expands ; and when 

 the air has reached the temperature of the surrounding 

 water, the piston will cease to ascend, and will remain sta- 

 tionary. But (2) if we suppose that when the heat is 

 applied, the piston is held down so as to keep the air under 

 a constant volume, an effort will be required to prevent the 

 piston from ascending in the tube, which becomes greater 

 in proportion as the air is heated. Hence 



(1) The effect of heat on a given quantity of air, the 

 elastic force remaining constant, is to expand its 

 volume. 



(2) Tlic effect of heat on a given quantity of air, the 

 volume remaining constant, is to increase its elastic 

 force. 



