260 Prof J. Dewar. [Mar. 9, 



10"S801 



This gives v c = — = 35 '08 and the critical density as 0*0285, 

 o *U 



a result midway between that got from Guldberg's ratio and that 

 derived from Young's reduced fluor-benzene. 



Further experimental results can be brought to bear on the question. 

 I have recently measured directly the heat of vaporisation of liquid 

 hydrogen at the boiling point and find it to be not more than 125 

 calories per gramme. Assuming the Eankine equation log 10 ^ = A - B/Y, 

 the molecular heat of vaporisation is 2B log e 10. Hence we get 

 B = 54-34783 ; and determining A from the values of p and t at the 

 boiling point we have in atmospheres, 



log 10 ^ = 54-34783 



1 1 



20-5 t 



Now, at the critical point we have found t c = 3 "04 p c , hence combining 

 these results we have finally, 



t c = 33*88°, and thence p c = 11-145 atmospheres. 



Before leaving these results we may write the Eankine equation in 

 Van der Waals's form, namely, 



, 54-34783 1-m 



_l0g € = , 



t e m 



where e and m are the reduced pressure and temperature, and Van 

 der Waals's /is 54*34783/4, or with the above result, 1*604, about half 

 the usual value of /. In like manner Trouton's constant, namely, 

 the ratio of the molecular latent heat to the absolute temperature 

 of the boiling point, is 125/20*5 or 6*096, again only about half the 

 usual value. 



In this connection we may refer to Olzewski's experimental observa- 

 tion of the temperature, 192° absolute, at which the Joule-Thomson 

 effect vanishes when the expansion is from a great pressure, in this 

 instance between 110 and 117 atmospheres. With the usual Van 

 der Waals's notation we may express the connection between this 

 inversion temperature and the critical temperature in either of the 

 forms, 



«. 



or 



(10), 



the former equation shows that for small pressures and consequent 

 values of v so great that b may be neglected in comparison with v, 

 the critical temperature is 4/27 of the temperature of inversion. But 



