LEROT C. COOLEY. 121 



in Regnault's apparatus. The compressibility of all rhn 

 others changed sign. Instead of steadily becoming more 

 compressible, as they had done below thirty atmospheres; 

 they became at one hundred less compressible, and their 

 deviations in this direction became larger and larger as 

 the pressure was still further augmented. 



This curious change in the sign of I he compressibility 

 of a gas at some high pressure has not been left without 

 confirmation. M. Cailletet has shown that, for air at 

 least, batterer's conclusion is essentially correct. The 

 change took place at about eighty atmospheres, and 

 thence up to six hundred its compressibility grew less 

 and less. 



We must now admit that there is one degree of press- 

 ure at which air conforms exactly to the requirements 

 of Boyle's law — a pressure of about eighty atmospheres 

 or twelve hundred pounds to the square inch. Accord- 

 ing to Natterer the same is true for several other gases. 

 Among those tried hydrogen is the only exception. 



But what is the effect of low pressures? Regnault's 

 experiments showed that the deviations from the law in- 

 creased rapidly with the increase of pressure. "Hence 

 we may conclude that as the pressure diminishes and the 

 gas expands, the deviation from the law of Marriotte 

 becomes gradually less, until at an infinite degree of ex- 

 pansion, this law would be the exact expression of the 

 truth."— {Coolie's Chem. Phys., p. 297). 



This is a most natural inference, and without waiting 

 for experimental confirmation it became the general be- 

 lief. Indeed an inference so well grounded as this would 

 appear to be, would seem to need no experimental evi- 

 dence to maintain it, nevertheless the investigation was 

 finally undertaken in Russia by Mendeleef. 



73 



