CHAPTER VIII 



GENERAL PROPERTIES OF GASES 



89. The Relation between Pressure and Volume. It was 



long known that as the pressure of a gas increases, that is, as 

 it becomes compressed, its volume decreases, but Robert 

 Boyle was the first to determine the exact relation between 

 the volume and the pressure of a gas. He did this by means 

 of the following simple experiment (Fig. 54). Let mercury 

 be poured in a U-shaped tube until the 

 level of the mercury in the closed end of 

 the tube is the same as the level of the 

 mercury in the open end. The mercury 

 just balances itself, and hence the air col- 

 umn in the closed end must be balanced 

 by the atmospheric pressure on the open 

 end ; the amount of the atmospheric pres- 

 sure can be easily determined by reading 

 the barometer. If the atmospheric pres- 

 sure as registered by the barometer is 30 

 inches or x inches, pour into the long arm 

 enough mercury to make the column D 

 equal to 30 or x inches. The air column 

 b is now under a pressure equal to that of 

 two atmospheres, one atmosphere of pres- 

 sure being due to mercury, the other atmosphere of pressure 

 being due to the weight of the atmosphere itself. If now the 

 air column in the closed end is measured, its volume will be 

 only one half of its former volume. By doubling the pres- 



95 



FIG. 54. As the pres- 

 sure on the gas in- 

 creases, its volume 

 decreases. 



