( 172 ) 



portance to the phenomenon. And in ttie second place the mist was 

 very faint indeed, which might point to a lower critical temperature 

 than had been derived. 



So it remained a very exciting question what the critical tem- 

 perature of helium would be. And in every direction in which 

 after the determination of the isotherms in hand we might try 

 to get more information about it, we were confronted by great 

 difficulties. 



As, however, they consisted in the arrangement of a cycle with 

 cooled helium, [this being indispensable to integrate cooling effects 

 with a reasonable quantity of helium] the labour spent for years on 

 the arrangement of the Leiden cascade of cycles for accurate 

 measurements, might contribute to the surmounting of them. Arrived 

 at this point I resolved to make the reaching of the end of the road 

 at once my purpose, and to try and effect the statical liquefaction 

 of helium with a circulation, as much as possible "corresponding" 

 to my hydrogen circulation. 



In this 1 perfectly realized the difficulty to satisfy at the same 

 time the different conditions for success [allowing for possible devi- 

 ations from the law of corresponding states]. For [though the suita- 

 bility of the hydrogen cycle for the cooling down of the compressed 

 helium to 15° K. was amply proved (Com m. N°. 103)] the preliminary 

 cooling to be reached was, as to the temperature, only just within 

 the limit at which it could be efficient, nor were the other circum- 

 stances which could be realized, any more favourable. 



Of course the scale on which the apparatus intended for the ex- 

 periment in imitation of the apparatus which had proved effective 

 for hydrogen, would be built, was not only chosen smaller in 

 agreement with the value of b which was put lower, but taken as small 

 as possible. That the reduction of Hampson's coil to smaller dimen- 

 sions does not diminish its action had been found by former expe- 

 riments, and has been very clearly proved by what Olszewski tells 

 about the efficiency of his small hydrogen apparatus. I could not, 

 however, reduce below a certain limit without meeting with con- 

 struction problems, about which the hydrogen apparatus had not 

 given any information. We had to be sure that the capillaries would 

 not get stopped up, that the cocks would work perfectly, that the 

 conduction of heat, friction etc. would not become troublesome. 

 When in connection with the available material, the smallest scale 

 at which I thought the apparatus still sufficiently trustworthy, reduc- 

 tion to half its size, had been fixed, the dimensions of the regene- 

 rator coil, though as small as those of Olszewski's coil, proved 



