232 Prof. J. Dewar. Application of Liquid Hydrogen to the 



inferred that no measurable pressure of air ought to be left in the 

 vessel. If we apply Van der Waal's law of corresponding temperatures 

 to the case of hydrogen, the above inference is made unimpeachable. 

 An approach to some knowledge of what the tension of air must be 

 about the boiling point of hydrogen can be attained by extrapolating 

 the vapour pressure curves of oxygen and nitrogen. Taking the follow- 

 ing range of boiling point temperatures for nitrogen and oxygen, viz., 

 from the critical point to the boiling point under diminished pressure, 

 two Willard Gibbs formulse were calculated, with the following 

 results : 



Nitrogen! 

 I 



abs ................ ?8 ' 6 59 



Pressure in mm ....... 25,900 740 26 



Nitrogen. Iog 10 p = 11-5561 __ 1-8980 log 10 T ...... (1). 



Oxygen /Temp, abs ................ 154 90'3 61-3 



I Pressure in mm ....... 37,592 740 7'5 



Oxygen. lo glo ^ * 9'4699 - - 0-9843 log 10 T. . . (2). 



Another Gibbs formula was calculated, taking Estreicher's values for 

 the vapour pressure of liquid oxygen below its boiling point, viz. : 



f Temp, abs .......... 91-44 78-1 62'8 



1 Pressure in mm. ... 743*8 141-8 7-5 



Oxygen. lo glo ^ = 16-0670-^11? - 3-8024 lo glo T ...(3). 



We deduce from these formulae the following vapour pressures at 

 the temperature of boiling hydrogen : 



(1) Nitrogen ............ 0-0015 Pressure in mm., 35 abs. 



(2) Oxygen ............ 0-000076 do. 



(3) ............ 0-000016 do. 



The results of calculation, taking the formulse for the widest range 

 of pressures, viz., (1) and (2), may probably be the surest, but in 

 any case those values must be taken as a maximum, seeing they 

 refer to the liquid state, while both oxygen and nitrogen, at the tem- 

 perature of 35 absolute, are hard solids, and must therefore have 

 dropped to lower tensions than that of the extrapolated liquid vapour 

 pressure curves. It is curious to note that at this low temperature the 

 theoretical ratio of the tensions of nitrogen and oxygen is as 20 to 1. 

 Direct measurements of the vapour pressure of nitrogen at the melting 



