﻿326 Pressure, Volume, and Temperature of Rarefied Gases. 



apparatus with nitrogen. The two values in each set have 

 been calculated with the two extreme values of the gauge ratio 

 that have been obtained during the experiments at this pressure 

 •6 millim. Though the results are not good, they nevertheless 

 show that the coefficient of expansion of nitrogen for one 

 degree Centigrade decreases with decrease of pressure. 



To sum up the results : — (1) The coefficient of expansion 

 of hydrogen with temperature decreases as pressure is lowered. 

 It is normal down to a pressure of Ol millim. (2) The co- 

 efficient of expansion of oxygen is greater than the normal 

 one, being 1/262 instead of 1/273 ; it increases with decrease 

 of pressure to 1/233 at 1*4 millim. pressure ; at 7 millim. 

 pressure it is erratic ; but at lower pressures it again becomes 

 more constant, still showing, however, a tendency to increase 

 as pressure is decreased. (3) With nitrogen the coefficient 

 of expansion is lower than the normal (1/304) at pressures 

 between 5 and 1 millim. ; at lower pressures, like that of 

 hydrogen, its coefficient of expansion decreases ; that is the 

 gas becomes more elastic. (4) So far as it was possible to 

 experiment with carbon dioxide, its behaviour appears to 

 resemble that of hydrogen and nitrogen ; but owing to the 

 tendency which it has to condense and cling to the gauge, 

 trustworthy measurements were impossible to attain. These 

 results corroborate those of Mendeleeff and Siljestrom, although 

 they are deduced from thermal expansion, while theirs were 

 deduced from the compressibility of the gns. And Bohr's 

 results as regards the abnormality of oxygen were also con- 

 firmed, although likewise by a different method. 



If it maybe taken for granted, then, that the rate of expansion 

 of gases decreases with decrease of pressure, it is necessary to 

 inquire why this is the case. From the point of view of the 

 kinetic theory, pressure is caused by impacts on the walls of 

 the containing vessel of the molecules which it contains, due 

 to their translational motion. The internal motion does not 

 give rise to pressure. If, on communicating to a gas energy, 

 by raising its temperature, it does not respond by a sufficient 

 rise of pressure, it appears to us that the conclusion is inevi- 

 table that its internal energy is increased to a greater than 

 usual extent compared with its translational energy. It is 

 perhaps idle to speculate on the extreme final state of rarefac- 

 tion ; but if this diminished rate of expansion were to continue 

 to increase with tenuity, a point might conceivably be reached 

 where all received energy would result in internal motion. 

 Can this be the cause, or one cause, of phosphorescence in 

 high vacua ? 



In his Allgemcine Cliemie, second edition, vol. ii. p. 32, 



