The Nadir of Temperature, and Allied Problems. 365 



boiling points. In other words, liquid helium at its boiling point 

 would have a refractive index of about the same value as liquid 

 hydrogen at its critical point, and as a consequence, small drops of 

 liquid helium forming in the gas near its critical point would be far 

 more difficult to see than in the case of hydrogen similarly situated. 



The hope of being able to liquefy helium, which would appear to have 

 a boiling point of about 5° absolute, or one-fourth that of liquid hydrogen f 

 is dependent on subjecting helium to the same process that succeeds with 

 hydrogen ; only instead of using liquid air under exhaustion as the 

 primary cooling agent, liquid hydrogen under exhaustion must be em- 

 ployed, and the resulting liquid collected in vacuum vessels surrounded 

 with liquid hydrogen The following table embodies the results of 

 experience and theory : — 



Initial temperature. 



Initial 

 temperature. 



Critical 

 temperature. 



Boiling points. 



52° C 



o 



5? 



15 



20 



75 

 325 

 760 



o 



2 ? 



6 



8 

 30 

 ]30 « 

 304 



o 



1 ? 



4 



5 (He ?) 

 20 (H) 

 86 (Air) 

 195 (C0 2 ) 



The first column gives the initial temperature before continuous 

 expansion through a regenerator, the second the critical point of the 

 gas that can be liquefied under such conditions, and the third the 

 boiling point of the resulting liquid. It will be seen that by the use 

 of liquid or solid hydrogen as a cooling agent we ought to be able to 

 liquefy a body having a critical point of about 6° to 8° absolute and 

 boiling point of about 4° or 5° absolute. Then, if liquid helium could 

 be produced with the probable boiling point of 5° absolute, this sub- 

 stance would not enable us to reach the zero of temperature ; another 

 gas must be found that is as much more volatile than helium as it is. 

 than hydrogen in order to reach within 1° of the zero of temperature. 

 If the helium group comprises a substance having the atomic weight 2,, 

 or half that of helium, such a gas would bring us nearer the desired 

 goal, In the meantime the production of liquid helium is a difficult 

 and expensive enough problem to occupy the scientific world for many 

 a day. 



A number of miscellaneous observations have been made in the 

 course of this inquiry, among which the following may be mentioned. 

 Thus the great increase of phosphorescence in the case of organic 

 bodies cooled to the boiling point of hydrogen under light stimula- 

 tion is very marked, when compared with the same effects brought 



