272 



PHYSIOLOGICAL PHYSICS. [Chap. xxui. 



they divide off. Into the long tube a small quantity 

 of mercury is poured, the tube being inclined. The 



mercury tills the bend B, and is 

 poured in till it stands at zero in 

 both limbs. The mercury thus 

 cuts off the air enclosed in the short 

 limb from communication with the 

 outside, and the equal level of the 

 mercury in both limbs shows that 

 the pressure exerted on the enclosed 

 air is equal to the external pres- 

 sure, i.e. the pressure of the atmo- 

 sphere. Mercury is then poured 

 in till, by its pressure, the enclosed 

 air is reduced to half its volume AD. 

 The added mercury gives the in- 

 crease of pressure. The air is 

 found to be reduced to half its 

 volume when the original pressure 

 is doubled, to one- third its volume 

 when the original pressure is 

 trebled, and so on ; that is, pressure 

 is inversely proportional to volume. 

 Other experimenters, Dulong and 

 Arago, increased the pressure 27 

 times, and found the law to hold 



good. 



But Regnault showed that 



it was not rigorously true, and that 



air, nitrogen, carbonic acid, and 

 ittj oxygen diminish in volume with 



increasing pressure more quickly 

 Fig ' n S u - Marriotte ' s than Marriotte's law allowed, while 



hydrogen is less compressible with 

 increasing pressures. 



Unequal compressibility. - Different gases 

 are unequally compressible. This was shown first of 

 all by Despretz in 1825, who took several cylindrical 



