Temperature of Hydrogen. 275 



cm. 3 



volume of hydrogen to be 23*45 — - and the critical density 



to be *043 -^-s . If these values be adopted as well as that 

 cm. 



of p c , supposed by Prof. Olszewski to be equal to 20 atm. 



(see below), then equation (3) gives 



£ c =41*3, i.e. approximately —232°; 



therefore the Law of Thermodynamic Correspondence requires 

 us to expect the critical temperature of hydrogen to be situated 

 in the proximity of —232°, the limits which the uncertainty 

 in the values of b allows for being —229° and — 234° 0. 

 respectively. 



If we assume —232° to be the critical temperature and 

 20 atm. to be the critical pressure of hydrogen, we may 

 calculate the boiling-point as follows. At that point the 

 " specific" value of the saturation pressure will be -^ or '05 ; 

 and the corresponding " specific" value of the absolute tem- 

 perature is (for all bodies) *706. Hence the boiling-point of 

 hydrogen will be —244°. Diminishing the pressure, the 

 following temperatures would be reached : — 



o 

 At 608 millim -244'5 



456 

 304 

 152 

 121-6 

 91-2 



At 60 8 millim -2493 



-245-3 



„ 30-4 



-2463 



n 15-2 



-2477 



n 7-6 



-248-1 



„ 3-8 



-248-6 



„ 15 



-2504 

 -251-4 

 -252-3 

 -253-2 

 -254-2 



Professor Olszewski has explained in the Philosophical 

 Magazine (for February 1895, pp. 202-203) how the experi- 

 ments he describes led him to the conclusion that the critical 

 pressure of hydrogen is 20 atm. Let us translate Prof. 

 Olszewski's reasoning into symbols. If we suppose for a 

 moment that the expansion (assumed to be adiabatic) which 

 begins with certain special values p and t of the pressure 

 and temperature, is just what is needed to carry the gas down 

 simultaneously to the critical temperature t c and the critical 

 pressure p c , then, to a first approximation, 



(s) - (£)" < 8 > 



(where the well-known ratio of specific heats, or about 14 for 

 hydrogen, is denoted by k). Substituting in (8) the data 



