238 Annals of the Philosophical Club 



hydrogen, and nitrogen can be calculated. Later investiga- 

 tion has confirmed the accuracy of the numbers for oxygen, 

 but for nitrogen the calculated value of the critical point was 

 50 too high. As hydrogen has not yet been liquefied, 1 it 

 may be inferred that its calculated critical point is above the 

 true value, which is confirmed by the fact that hydrogen 

 does not liquefy at the calculated critical temperature, which 

 is about 174 C. The increasing inaccuracy in the cases 

 of nitrogen and hydrogen, compared with the accuracy 

 in that of oxygen, is due to the much lower value of their 

 respective critical points, and to the fact that, as the iso- 

 thermal curves are observed at ordinary temperatures 

 about 15 C. they are being gradually further removed 

 from the critical points. Hence, isothermals must be 

 observed at much lower temperatures to obtain anything 

 like an accurate calculated value for hydrogen. Wroblewski 

 constructed such curves, and they show the critical point of 

 hydrogen to be 240-4 C. (or 32-6 absolute with a 

 critical pressure of 13-3 atmospheres), the density at the 

 contact point being 0-027 '> both critical point and density are 

 below those of any other substance. Calculated from these 

 data the boiling point of hydrogen, at ordinary atmospheric 

 pressure, would be between 20 and 22 absolute temperature, 

 the difference between critical and boiling point being about 

 10, while it is about 50 in the case of nitrogen and carbonic 

 acid. So if we had liquid hydrogen, we should have to lower 

 its boiling point 20 to reach the zero of absolute temperature. 

 As such reduction can be effected when oxygen, nitrogen, 

 and other substances are evaporated under diminished 

 pressure, there is no apparent reason for doubting that, with 

 a supply of liquid hydrogen, we could arrive at, or very near 

 to, the zero of absolute temperature. Before this could be 

 reached, hydrogen would be a solid mass, probably having 

 the appearance and characters of a metal. 



1 Its critical temperature (-233C.) and the boiling point (-243C.) 

 were ascertained by Professor Olszewski (Nature, vol. li. page 488). He 

 also obtained a momentary liquefaction of the gas, but Professor Dewar 

 himself succeeded on May ioth, 1898, in reducing the gas to a static liquid. 

 For an account see Nature, vol. Iviii. page 49. 



