HISTORY OF COLD AND THE ABSOLUTE ZERO. 227 



giis is ji pro])l(Mn for the futur(\ this does not prevent us from antiei- 

 ])at'mi;- some of th(^ pnyperties of the liuid body. It would ))e twice 

 as dense as liquid liydroj^-eu if the ratio of the critical constants has 

 the same value as in the case of h^'drogen — that is to say, the critical 

 ])ressure will not exceed 4 or 5 atmospheres. The liijuid would on 

 this assumption possess a very feeble surface tension, and its compress- 

 ibility and expansibility would be about four times that of liipiid 

 hydrogen, while the heat re({uired to vaporize the molocule would be 

 a1)out one-fourth that of li({uid hvdrog-en. If the critical pressure 

 should turn out to ]>e as high as that of nitrogcni or oxygen, th(>u the 

 iluid density would exceed that of water, and the surface tension 

 increased, while the compressibility would be diminished. Heating 

 th(^ li({ui(l 1 a))Ove its boiling point woidd raise the pressure b}" If 

 atmospheres, which is more than four times the increment for li([uid 

 hydrogen. The liquid would be only seventeen times denser than its 

 vapor, whereas li({uid hydrogen is sixty-five times denser than the gas 

 it gives oti'. Only some S'-* or -i would separate the critical tempera- 

 ture from the l)oiling point and the melting point, whereas in licpiid 

 hydrogen the separation is respectively lo- and 15"'. As the li([uid 

 refractivities for oxygen, nitrogen, and hydrog(Mi are closely propor- 

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

 helium has only one-fourth the refractivity of hydrogen, although it 

 is twice as dense, we may infer that the refractivit}' of liquid helium 

 would also be about one-fourth that of liipiid h\"drogen, uidess the 

 critical pressure is high, wdiich would necessitate an increase in the 

 value. Now hydrogen has the smallest refractivity of anv known 

 licpud. and yet li<}ui(l helium will have only about one-fourth of this 

 value — comparable in fact with liquid hydrogen just below its crit- 

 ical point. This means that the licpud will be quite exceptional in its 

 optical pro])erties, and very difficult to see. This may be the explana- 

 tion wdiy no mist has been seen on its adiabatic expansion from tlie low- 

 (\st temperatures. Taking all these remarkal)le properties of the liquid 

 into consideration, one is afraid to predict that we are at present able 

 to cope with the difficulties involved in its producti(.)n and collection. 

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

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

 ing a change of state in hydrogen through the us(^ of li(piid aii', we 

 may safeh' predict that helimn can be liquefied l)y following similar 

 methods. If, however, the critical point is as low as 6° absolute, then 

 it would be almost hopeless to anticipate success b}" adopting the pro- 

 cess that works so well with hydrogen. The present anticipation is 

 that the gas will succuml) aftei l)eing subjected to this process, only, 

 instead of liquid air under exhaustion being used as the primary cool- 

 ing agent, liquid hydrogen evaporating under similar circumstances 

 must be employed. In this case the resulting liquid would require to 



