August 20, 1908 



NA TURE 



371 



work at Leyden, there has been continuous effort to 

 reach the nadir of temperature. It has taken some 

 years to get the necessary data for helium together. 

 In 1905 much help was obtained from the Commercial 

 Intelligence Oflico at Amsterdam under the direction 

 of Mr. O. Kamerlingh Onnes, who obtained a suffi- 

 ciency of the monazite sand, used for the prepara- 

 tion of helium, at a cheap rate. The helium is ob- 

 tained from this by heat, and is then most carefully 

 purified. The first isotherm determinations on helium 

 were made in 1907. It was owing to Olszewski's 

 and Dewar's failures that various methods, such as 

 the helium motor with vacuum glasses as cylinder 

 and piston, were considered, but these were aban- 

 doned, owing to the results of the isotherm deter- 

 minations, which pointed to a critical temperature of 

 about 5° to 6° K. This result was in better agree- 

 ment with Dewar's estimate of 8° K. obtained from 

 experiments of absorption by charcoal than with 

 Olszewski's of below 2° K. 



However, the conclusion from the isotherms was 

 not quite decisive, as those at the lowest temperature 

 indicated a lower critical temperature than those at 

 higher temperatures, and this appeared to throw some 

 doubt upon the strict applicability of the law of corre- 

 sponding states to helium. At all events, just before 

 the experiment was undertaken it was shown that 

 the Boyle point, though below the boiling point of 

 hvdrogen, was somewhat above 15° K., which is 

 obtainable with liquid hydrogen under reduced 

 pressure. 



The time had hence arrived to reap the fruit of 

 the many years of work devoted to building up the 

 cryogenic laboratory for the use of prolonged accurate 

 measurements in liquid gases, with all the circu- 

 lations so arranged that the gases remain pure. 

 This is particularly important in the hydrogen cycle, 

 where 4 litres of liquid can be dealt with per hour 

 and a supply can be obtained in a state of great 

 purity and stored for use. 



In the arrangement of the experiment constant use 

 was made of the theory of van der Waals. The 

 apparatus was made as small as possible, but there 

 was a practical limit which was fixed by its neces- 

 sary relation to the other apparatus in use. To com- 

 press the helium the special mercury pump was used 

 which was completed in 1S88, and was used to give 

 baths of static oxygen in 1894. It compresses to 

 100 atmospheres, which, with the critical pressure 

 below 5 atmospheres for helium, is a high reduced 

 pressure. This pump circulates 1400 litres per hour, 

 which is sufficient with the dimensions of the appa- 

 ratus taken, and has a capacity with connections of 

 about 200 litres. For this experiment it was not 

 possible to run the helium and hydrogen cycles at the 

 same time, so that sufficient liquid hydrogen had to 

 be made before the experiment on helium was com- 

 menced. However, now that the main difificulties are 

 overcome, it will be possible to work the two cycles 

 simultaneously. 



In directing attention to Sir J. Dewar's work for 

 this and similar researches Prof. Onnes points out 

 especially the use he has made of the selective absorp- 

 tion of charcoal for gases in the purification of gases 

 under pressure, and to the advantage of silvered 

 vacuum glasses. 



Such glasses are used at every stage of the work. 

 For example, the liquid hydrogen is collected in an 

 unsilvered glass placed in liquid air contained in a 

 silvered glass with a strip of clear glass left to enable 

 the interior to be seen. The liquid hydrogen is trans- 

 ferred by pressure through a fine tube into the 

 experimental apparatus. 



A detailed description is given in the paper of this 



NO. 2025, VOL. 78] 



apparatus, which is, however, simple enough in prin- 

 ciple. The regenerator spiral, through which the 

 compressed helium is expanded, is contained in the 

 upper part of a vacuum glass also containing lower 

 down the upper bulb of a helium thermometer. The 

 helium glass is contained in a second which is filled 

 with liquid hydrogen and connected to the hydrogen 

 circulation. This glass in turn is contained in 

 another filled with liquid air, and this finally in one 

 containing alcohol. All these glasses are unsilvered, 

 so that a clear view is obtained of the central glass 

 and its contents. 



The day before the successful experiment, July g, 

 was devoted to the preparation of 75 litres of liquid 

 air, and at 5.45 a.m. on July 10 the work was com- 

 menced to obtain the necessary liquid hydrogen. By 

 1.30 p.m. 20 litres were standing in the special 

 vacuum glasses. Meanwhile the helium and hydro- 

 gen circulations were pumped free of air and washed 

 through with their respective gases, and a start was 

 made to cool the liquid-air glass. At 2.30 hydrogen 

 cooled by liquid air was taken through the hydrogen 

 glass, and by 3 p.m. the temperature was down to 

 — 180° C. At 4.20 the helium circulation was 

 started, liquid hydrogen was introduced into its glass, 

 and the pressure lowered until at 5.20 p.m. it reached 



6 cm., at which it was kept. Between 5.30 and 6.30 

 the pressure of helium in the spiral was gradually 

 raised to 100 atmospheres. At 6.35, when the pres- 

 sure was allowed to fall rapidly to 40 atmospheres, 

 the helium thermometer indicated a temperature 

 below that of the liquid hydrogen ; nearly 6° K. was 

 read once. At this time the last reserve of liquid 

 hydrogen was connected, and no liquid helium had been 

 seen. A quicker expansion was allowed, and the 

 temperature fell and constantly returned to the same 

 temperature of less than 5° K. It was as though 

 the thermometer stood in liquid. 



Somewhat later, at about 7.30, the surface was 

 seen at the top of the vacuum glass. The liquid 

 having been found under ordinary pressure there was 

 no doubt that the critical pressure was more than 

 I atmosphere. The surface was illuminated from 

 below, and had the appearance of a liquid 

 near the critical state in a Cagniard de la 

 Tour tube, cutting the walls like knife-edges, 

 though in this case the diameter was 5 cm. 

 There was also a marked contrast between the 

 helium and the hydrogen in the next outer tube. 

 Some of the evaporated helium was now collected 

 and used for a density determination giving 2'oi. 

 At 8.30 the pressure on the helium was reduced, and 

 2'3 cm. was measured. The pumps, however, can 

 give 2 mm., and it is quite possible that as little as 



7 mm. was reached, but no solid could be seen. At 

 9.40 only a few c.c. of liquid helium remained. Thus 

 liquid helium, starting with an amount exceeding 

 60 c.c, had been under observation for more than 

 two hours. 



All the evaporated helium was collected into three 

 portions, which gave densities of 2'o4, I'gg, and 2'o2. 

 As a further test of purity a special comparative 

 spectroscopic investigation was made with known 

 mixtures of hvdrogen with heliurn, and it proved 

 that not more than o'oo8 per cent, hydrogen was 

 present. This high degree of purity is also con- 

 firmed by the easy working of all cocks, which would 

 have been stopped by a very little frozen hydrogen, 

 and also by the condition of the last remaining 

 liquid. The thermometer was also controlled by a 

 measurement of the boih'ng point of oxygen, which 

 gave 89° K. instead of 90° K. 



The properties found are as follows : — A boiling 

 point of 4°'3 K. on a constant volume helium ther- 



