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1.5 liter into the ciyostat (described in VIII of the series "Methods 

 and apparatus used in the Cryogenic Laboratory" of these commu- 

 nications) and to keep it to within 0°.01 at a uniform and constant 

 temperature. The requirements were therefore very much higiier than 

 they had formerly been for the bath of liquid oxygen. These require- 

 ments could by no means be fulfilled before I had the disposal of 

 a vacuum pump (mentioned as early as Jan. '96 in Comm. N". 23), 

 (comp. Comm. N°. 83, March '03), suitable to evaporate in a short 

 time large quantities of liquid air at a pressure of a few centimeters, 

 and before I possessed comjiressors for constant working with ex- 

 tremely pure hydrogen. With the former instrument and the com- 

 pressors, described in § 3, the liquefactor, described in § 2, delivers 

 3 a 4 liters of liquid hydrogen per hour. Thus T was able to bring 

 to this assembly (28 May '06) 4 liters of liquid hydrogen prepared 

 at Leiden the day before and to use it in several experiments. 



Our installation proved quite satisfactory for operations with the 

 afore mentioned cryostat. After we had succeeded in making with 

 it some measurements in liquid hydrogen boiling under ordinary 

 and under reduced pressure the vacuum glass of the cryostat cracked 

 and only by mere accidence the measuring apparatus were spared. 

 Therefore we have constructed another modified cryostat, to be 

 described in XII, which besides insuring the safety of the measuring 

 apparatus has the advantage of using less liquid hydrogen than the 

 cryostat, described in VIII (Comm. N°. 94'^, June '05). This new 

 cryostat entirely satisfies the requirements ; the temperature is kept 

 constant to within 0^,01. It is noteworthy that while the measure- 

 ments are being made the cryostat shows in no way that we are 

 working with a bath of no less than 1.5 liter of liquid hydrogen. 



I wish to express thanks to Mr. G. J. Flim, mechanist at the 

 cryogenic laboratory, for his intelligent assistance. Under his super- 

 vision the liquefactor and cryostat, to be described in the following 

 sections, and also other accessories have been built upon my direc- 

 tion in the workshop of the laboratory. 



^ 2. The hydrogen liquefactor for constant use. 



a. The apparatus does not yet entirely realize the original design '). 



1) It might be improved by dividing the regenerator spiral in several successive 

 coils, each opening into the next with its own expansion-cock, where the pressures 

 are regulated according to the temperatures. Compare the theory of cooling with 

 the Joule-Kelvin process and the liquefying by means of the Linde process given 

 by van der Waals in the meeting of Jan. 1900. 



