LE ROY 0. COOLEY. 123 



had to be kept constant, and the readings of volumes 

 and hights had to be taken with extreme precision. 



The pressures employed lay between about eighty-five 

 hundredths and twenty-six thousandths of an atmos- 

 phere, or more exactly between 650 mm. and 20 mm. of 

 mercury column. 



The gases examined were air, hydrogen, carbon diox- 

 ide, and sulphurous oxide, with the following results : 



Hydrogen, which had proved to be less compressible 

 under great pressures than it should be according to 

 Boyle's law, maintained that characteristic to the end ; 

 but, contrary to the prediction, its departures became 

 greater and greater as the pressures diminished. In 

 fact, its deviation from the law was five times greater 

 when the pressure was reduced to 120 mm. than when 

 the greater pressure of 400 mm. was upon it. 



Air, as the pressure ran down from one atmosphere to 

 600 mm., was found to remain more compressible, as it 

 had been at higher pressures. But at about that point 

 the sign of the deviation changed, and, like hydrogen, 

 it became less and less compressible, thenceforth, with 

 increasing deviations from the law. 



For the other gases the same change of sign occurred. 

 Carbon dioxide, for example, varies from the law, its 

 compressibility being too great —twenty-nine parts in 

 ten thousand — when the pressure diminishes from 

 635 mm. to 200 mm., while, carrying the pressure still 

 lower — from 190 mm. to 22 mm. — the same gas departs 

 from the law the other way to the extent of seventeen 

 parts in ten thousand. 



At a certain degree of low pressure, all these gases, 

 except hydrogen — which is already so — change from 

 being more compressible to being less compressible than 

 if Boyle's law were rigorously true ; and we must admit 

 that at this pressure, determinate for each gas, the law 

 is an exact expression of the truth. When rarefied be- 



75 ' 



