114 THE PRODUCTION AND USE OF HEAT 



131. Effect of Heat upon the Volume of Air. 

 Exercise 35. Air Expanded by Heat 



Fit a 10- or 12-in. glass tube into the stopper and the stopper into 

 the flask. Be sure that the apparatus is air-tight (Fig. 87). Invert 

 the flask so that the end of the tube dips into the water in a vessel. 

 Gently apply heat to the flask, constantly turning it so as to heat it 

 evenly on all sides. Do bubbles of air escape from the tube? Heat 

 the flask quite hot ; then remove the flame and allow the flask to cool. 

 What happens? What portion of the air was forced out of the flask? 



When air is heated, it always expands. When a certain 

 volume of air at the temperature of freezing water is heated 

 to the temperature of boiling water, it 

 increases nearly J-<j in volume. If it is 

 heated but 1C., it increases exactly H?3 

 part of itself. This fact was discovered 

 by a Frenchman named Charles in 1787. 

 He discovered that this was the rate at 

 which all gases expand when heated. 

 This fact is called CHARLES' LAW and is 

 stated thus: Pressure remaining constant, 

 the volume of a given portion of gas in- 



FIG. 87.~Effect of heat CTeaS6S % P art f itS volume at zero 



upon volume of air. centigrade for each rise of 1C. above that 

 temperature, and it decreases ^73 of its 

 volume for each fall of 1 below that temperature. Were we 

 to state the law in the Fahrenheit scale of temperature, we 

 should use 32F. in the place of 0C. and the fraction K 91 in 

 the place of M?3- Explain why this is so (see Art. 22). 



132. Application of Charles' Law. It is not probable that 

 in any two lamps we might examine we should find that the 

 gases within the chimney are heated to exactly the same 

 temperature. But we are probably not far from the truth 

 if we say that the gases within the ordinary lamp chimney are- 

 so heated that they are expanded to twice the volume they 

 had when they entered the bottom of the burner. Every 

 cubic inch of air which enters the burner leaves the top of 

 the chimney as 2 cu. in., if this be true. The same thing takes 



