310 Scientific Intelligence. 



helium will be four times more v^olatile than liquid hj^drogen, 

 just as liquid hydrogen is four times more volatile than liquid 

 air. Although the liquefaction of the gas is a problem for the 

 future, this does not prevent us from safely anticipating some of 

 the properties of the fluid body. It would be twice as dense as 

 liquid hydrogen, with a critical pressure of only 4 or 5 atmos- 

 pheres. The liquid would possess a very feeble surface-tension, 

 and its compressibility and expansibility would be about four 

 times that of liquid hydrogen, while the heat required to vaporize 

 the molecule would be about one-fourth that of liquid hydrogen. 

 Heating the liquid 1 degree above its boiling-point would raise 

 the pressure If atmospheres, which is more than four times the 

 increment for liquid hydrogen. The liquid would be only seven- 

 teen times denser than its vapor, whereas liquid hydrogen is sixty- 

 five times denser than the gas it gives off. Only some 3 or 4 

 degrees would separate the critical temperature from the boiling- 

 point and the melting-point, whereas in liquid hydrogen the sepa- 

 ration is respectively 10 and 15 degrees. As the liquid refractivi- 

 ties for oxygen, nitrogen and hydrogen are closely proportional 

 to the gaseous values, and as Lord Rayleigh has shown that 

 helium has only one-fourth the refractivit^^ of hydrogen, although 

 it is twice as dense, we must infer that the refractivity of liquid 

 helium would also be about one-fourth that of liquid hydrogen. 

 No«^ hydrogen has the smallest refractivity of any known liquid, 

 and yet liquid helium will have only about one-fourth of this 

 value — comparable, in fact, with liquid hydrogen just below its 

 critical point. This means that the liquid will be quite excep- 

 tional in its optical properties, and very difficult to see. This 

 may be the explanation of why no mist has been seen on its adia- 

 batic expansion from the lowest temperatures. Taking all these 

 remarkable properties of the liquid into consideration, one is 

 afraid to predict that we are at jjresent able to cope with the 

 difficulties involved in its production and collection. Provided 

 the critical point is, however, not below 8 degrees absolute, then 

 from the knowledge of the conditions that are successful in pro- 

 ducing a change of state in hj^drogen through the use of liquid 

 air, we may safely predict that helium can be liquefied b}^ follow- 

 ing similar methods. If, however, the critical point is as low as 

 6 degrees absolute, then it would be almost hopeless to anticipate 

 success by adopting the process that works so well with hydrogen. 

 The present anticipation is that the gas will succumb after being 

 subjected to this process, only, instead of liquid air under exhaus- 

 tion being used as the primary cooling agent, liquid hydrogen 

 evaporating under similar circumstances must be employed. In 

 this case, the resulting liquid would require to be collected in a 

 vacuum vessel the outer walls of which are immersed in liquid 

 hydrogen. The practical difficulties and the cost of the operation 

 will be very great ; but, on tlie other hand, the descent to a 

 temperature within 5 degrees of the zero would open out new 

 vistas of scientific inquiry, which would add immensely to our 



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