

EXPERIMENTS ON MARIOTTE'S LAW. 



243 



if the pressure is doubled, the volume of the enclosed 

 air is diminished to one half; and that if the pressure 

 is reduced to one half, the volume is doubled. If 

 experiments are made at various pressures, the general 

 law is proved that : The volume of a gas decreases in 

 the same proportion as that in which the pressure upon it 

 increases, and it increases in the same proportion as that 

 in which the pressure decreases; or, more briefly, The pres- 

 sure and volume of a gaseous mass are inversely propor- 

 tional. 



This law is usually called Mariettas Law ; it holds 

 good for all c permanent ' gases, that is, those which 

 have not yet been liquefied by pressure, as oxygen, 

 nitrogen, atmospheric air, etc. But some gases for 

 example, carbonic acid, ammoniacal gas, etc. are 

 c coercible,' that is, they may be liquefied by pressure; 

 such gaseous bodies manifest considerable deviations 

 from the law. 



The following experiments on Mariotte's law, though less striking, 

 are more easily performed than those just described. 



I 



1 67 (~ nal size). 



Take a glass tube, the same as that used for the barometer experi- 

 ment (p. 232, fig. 161), provided its bore is of pretty uniform width 

 throughout, Measure off 10 cift from the open, and 20 cm from the 



R 2 



