and Allied Problems. 171 



temperature of the expanding gas was twice the critical tem- 

 perature, but it was not visible when the initial temperature 

 was about two and a-half times the critical temperature. This 

 experience applied to interpret the helium experiments, would 

 make the critical temperature of the gas under 9° absolute. 



Olszewski in his experiments expanded helium from about 

 seven times the critical temperature under a pressure of 125 

 atmospheres. If the temperature is calculated from the adia- 

 batic expansion starting at 21° absolute, an effective expansion 

 of only 20 to 1 would reach 6°% and 10 to 1 of 8°*3. It is 

 now safe to say, helium has been really cooled to 9° or 10° 

 absolute without any appearance of liquefaction. There is 

 one point, however, that must be considered, and that is the 

 small refractivity of helium as compared to hydrogen, which, 

 as Lord Kayleigh has shown, is not more than one-fourth the 

 latter gas. j\ t ow as the liquid refractivities are substantially 

 in the same ratio as the gaseous refractivities in the case of 

 hydrogen and ox\ T gen, and the refractive index of liquid 

 hydrogen is about 1*12. then the value for liquid helium 

 should be about 1*03, both taken at their respective 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 criti- 

 cal 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, 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 

 employed, and the resulting liquid collected in vacuum vessels 

 surrounded with liquid hydrogen. The following table em- 

 bodies the results of experience and theory : — 



T -,. i , Initial Critical D .,. - , 



Initial temperature. . , , Boihno' points. 



1 temperature, temperature. sr 



Liquid helium? 5°? 2°? 1°? 



Solid hydrogen 15 6 4 



Liquid •' 20 8 5 (He?) 



Exhausted liquid air 75 30 20(H) 



52° C 325 130 86 (Air) 



Low Red Heat 750 304 195 (C0 2 ) 



The first column gives the initial temperature before con- 

 tinuous expansion through a generator, 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 



Am. Jour. Sci. — Fourth Series, Vol. XII, No. 68. — August, 1901. 

 12 



