116 COMPEESSIBILITY OF GASES. 



of the reservoir was a solid metallic plunger which could 

 be thrust still further by means of a powerful screw. 



Equal vol umes of two gases, dry and pure, introduced 

 into the tubes could be submitted to enormous pressures 

 by this screw plunger whose descent would drive the 

 mercury against them, while the great lengths and small 

 diameters of the gas columns would permit the changes 

 of volume to be distinctly seen even when the volumes 

 of the gases would be reduced to a small fraction of 

 their original values. 



Pouillet's apparatus fully confirmed the results of 

 Despretz's experiments. It showed that no two gases 

 suffered compression exactly alike, and that hydrogen 

 alone was less compressible than air. 



DULONG AND AEAGO 1 — 1829. 



Despretz's experiments did not question the truth of 

 Boyle's law in its application to air. For this substance 

 and for its constituents — nitrogen and oxygen — the law 

 was still supposed to hold good. Bat when it is remem- 

 bered that this ^opinion had no better foundation than 

 the fact that there were obvious sources of error in the 

 experiments which exhibited the deviations of air it will 

 be seen that the attitude of this substance toward the 

 law was still an open question. 



Dulong and Arago, in 1829, submitted the question to 

 an experimental test. Their apparatus was essentially 

 the same as that used originally by Boyle, but with 

 such modifications and refinements as would secure 

 greater accuracy and admit the use of vastly greater 

 pressures. 



The tube which enclosed the air was about six feet 

 high, an eighth of an inch in diameter inside, and accu- 



lMemoires de I'Academie des Sciences, torn. x. 

 Annales de Chemie et de Physique, 3d series, torn, xliii. 

 Ganot's Physics, Atkinson, 8th ed., p. 135. 

 Cooke's Chem. Physics, p. 293. 



es 



