October 10, 1902.] 



SCIENCE. 



577 



uted, like hydrogen, in the sun, stars, and 

 nebiiliB. A specimen of this gas was sub- 

 jected by Olszewski to liquid air tempera- 

 tures, combined with compression and sub- 

 sequent expansion, following the Cailletet 

 method, and resiilted in his being unable 

 to discover any appearance of liquefaction, 

 even in the form of mist. His experiments 

 led him to infer that the boiling-point of 

 the substance is probably below nine de- 

 grees absolute. After Lord Rayleigh-had 

 found a new source of helium in the gases 

 which are derived from the Bath springs, 

 and liquid hydrogen became available as a 

 cooling agent, a specimen of helium cooled 

 in liquid hydrogen showed the formation 

 of fluid, but this turned out to be owing to 

 the presence of an unknown admixture of 

 other gases. As a matter of fact, a year 

 before the date of this experiment I had 

 recorded indications of the presence of 

 unknown gases in the spectrum of helium 

 derived from this source. When subse- 

 quently such condensable constituents were 

 removed, the purified helium showed no 

 signs of liquefaction, even when compressed 

 to eighty atmospheres, while the tube con- 

 taining it was surrounded with solid hy- 

 drogen. Further, on suddenly expanding, 

 no instantaneous mist appeared. Thus 

 lelium was definitely proved to be a much 

 more volatile substance than hydrogen in 

 •either the liquid or solid condition. The 

 inference to be drawn from the adiabatie 

 expansion effected under the circumstances 

 is that helium must have touched a tempe- 

 rature of from nine to ten degrees for a 

 short time without showing any signs of 

 liquefaction, and consequently that the 

 critical point must be still lower. This 

 would force us to anticipate that the boil- 

 ing-point of the liquid will be about five 

 degrees absolute, or liquid helium will be 

 four times more volatile than liquid hydro- 

 gen, 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 prop- 

 erties of the fluid body. It would be twice 

 as dense as liquid hydrogen, with a critical 

 pressure of only four or five atmospheres. 

 The liquid would possess a very feeble sur- 

 face-tension, and its compressibility and 

 expansibility would be about four times 

 that of liquid hydrogen, while the heat re- 

 quired to vaporize the molecule would be 

 about one fourth that of liquid hydrogen. 

 Heating the liquid one degree above its 

 boiling-point would raise the pressure by 

 one and three fourth atmospheres, wliich is 

 more than four times the increment for 

 liquid hydrogen. The liquid would be only 

 seventeen times denser than its vapor, 

 whereas liquid hydrogen is sixty-five times 

 denser than the gas it gives off. Only 

 some three or four degrees would separate 

 the critical temperature from the boiling- 

 point and the melting-point, whereas in 

 liquid hydrogen the separation is respec- 

 tively ten and fifteen degrees. As the 

 liquid refractivities 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 

 refractivity of hydrogen, although it is 

 twice as dense, we must infer that the re- 

 fractivity of liquid helium would also be 

 about one fourth that of liquid hydrogen. 

 Now hydrogen has the smallest refractiv- 

 ity 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 

 explanation of why no mist has been seen 

 on its adiabatie expansion from the lowest 

 temperatures. Taking all these remarkable 

 properties of the liquid into consideration, 

 one is afraid to predict that we are at 



