LEROY 0. COOLEY. 115 



whose compressibility he intended to compare with 

 that of air as a standard. Thus arranged he placed the 

 tubes and cistern in a strong glass cylinder completely 

 filled with water and having a screw piston through the 

 top by means of which pressure could be applied and 

 greatly augmented. 



Under these conditions the several gases were simul- 

 taneously brought under identical pressures and any 

 difference in behavior could be readily detected. Ob- 

 serving the air and hydrogen tubes, the mercury rose in 

 them equally until the pressure reached fifteen atmos- 

 pheres, but when it surpassed that amount the mercury 

 Tose more slowly in the hydrogen tube, showing that hy- 

 drogen is less compressible than air. Sulphuretted hydro- 

 gen, ammonia, cyanogen and carbon dioxide, submitted 

 to the same treatment showed still larger variations, and 

 at much lower pressures. But unlike that of hydrogen 

 the compressibility of all these gases was greater than 

 that of air. 



In fact each gas at high pressures exhibited a com- 

 pressibility peculiar to itself. Hence Boyle's law could 

 not be universal in its application. If it be rigorously 

 true for air it must be untrue for all other gases. 



pouillet. 1 



The unequal compressibility of gases was soon after- 

 ward verified by Pouillet, with an apparatus which per- 

 mitted the use of still greater pressures. His gas tubes 

 were about six feet high, one-tenth of an inch internal 

 diameter and carefully calibrated. Their feet were firmly 

 fastened into a cast iron vessel. The bottom of this ves- 

 sel communicated, by an iron tube, with a cast iron reser- 

 voir containing mercury surmounted by oil, which com- 

 pletely filled it. Projecting into the oil through the top 



1 Elements de Physique, Pouillet, 5th ed., torn. i. 

 Deschanel Natural Phil., Everett, p. 173. 



87 



