( 174 ) 



temperature per hour, */» of the displacement with the hydrogen 

 circulation. Not before experiments with the latter had been made, 

 in which the preliminary cooling of the hydrogen did not take place 

 with air evaporating at the vacuum pump (so at — 205°) but under ordi- 

 nary pressure (so at — 190°), and moreover the hydrogen compressor 

 ran 4 times more slowly than usual, and in these experiments liquid 

 hydrogen had yet been obtained, it might be assumed that the cir- 

 culation process to be realized would still be sufficient to accumulate 

 liquid helium. 



With regard to the parts of the compressors, the auxiliary apparatus, 

 and the conduits, which in the course of the experiment assume the 

 same pressure as the regenerator coil, their joint capacity was small 

 enough to enable us to make the experiment with a quantity of 200 

 liters. This quantity of pure helium besides a certain quantity [160 

 liters] kept in reserve could be ready within not too long a time *). 



A great difficulty of an entirely different nature than the preceding 

 one consisted in this that the hydrogen circulation and the helium 

 circulation could not be worked simultaneously with the available 

 helpers to work them. It is true that the two circulations have been 

 arranged not only for continuous use, but if there is a sufficient 

 number of helpers, also for simultaneous use, but in a first experiment 

 it was out of the question to look, besides after the helium circulation, 

 also after the hydrogen circulation, the working of which requires 

 of course, great experience -). So on the same day that the helium 

 experiment was to be made, a store of hydrogen had to be previously 

 prepared large enough to provide for the required cooling during 

 the course of the helium experiment. It was again the law of corre- 

 sponding states which directed us in the estimation of the duration 

 of the experiment and the required quantity of liquid hydrogen 3 ). 

 They remained just below the limit at which the arrangement of 



1 ) [That success was only possible by applying the cycle method is evident from 

 the fact that the helium lias passed the valve 20 times before liquefaction was 

 observed, and the considerable labour expended on the preparation of the pure 

 helium would have been increased in the same proportion i.e. to an extravagant 

 amount]. 



2 ) [Now the great difficulties of a first liquefaction have been overcome simul- 

 taneous working has become possible, though it remains the question how to 

 find the means to develop the laboratory service according to the extension of its 

 field of research]. 



8 ) [The hydrogen cycle is not only arranged so that the same pure hydrogen in 

 it can be circulated and liquefied at the rate of 4 liters per hour as long as this 

 is wished, but also allows (as will be treated in a following communication) easily 

 to prepare great stores of extremely pure hydrogen gas, which can be tapped off 

 from the apparatus as liquid at the rate of 4 liters per hour]. 



