ADDRESS. 37 



rature of from 9 to 10 degrees for a short time vvithout showing any signs of 

 liquefaction, and consequently that the critical point must be still lower. 

 This would force us to anticipate that the boiling-point of the liquid will 

 be about 5 degrees absolute, or liquid helium will be four times more 

 volatile than liquid hydrogen, just as liquid hydrogen is four times more 

 volatile than liquid air. Although the liquefaction of the gas is a pro- 

 blem for the future, this does not prevent us from anticipating some 

 of the properties of the Huid body. Tt would be twice as dense as liquid 

 hydrogen if the ratio of the critical constants has the same value as 

 in the case of hydrogen — that is to say, the critical pressure will not 

 exceed 4 or 5 atmospheres. The liquid would on this assumption 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 vaporise the molecule would be about one-fourth that of liquid hydro- 

 gen. If the critical pressure should turn out to be as high as that of 

 nitrogen or oxygen, then the fluid density would exceed that of water 

 and the surface-tension be increased, while the compressibility would be 

 diminished. Heating the liquid 1 degree above its boiling-point would 

 raise the pressure by 1 1 atmosphere, which is more than four times the 

 increment for liquid hydrogen. The liquid would be only seventeen 

 times denser than its vapour, whereas liquid hydrogen is sixty-tive 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 separation is respectively 10 and 

 15 degrees. As the liquid refractivities for oxygen, nitrogen, and hydro- 

 gen are closely proportional 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 refractivity of 

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

 unless the critical pressure is high, svhich would necessitate an increase 

 in the value. Now 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 exceptional in 

 its optical properties, and very difficult to see. This may be the ex- 

 planation of why no mist has been seen on its adiabatic expansion from 

 the lowest temperatures. Taking all these remarkable 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 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 hydrogen through the use of liquid air, we 

 may safely predict that helium can be liquefied by following 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 pro- 

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



