( 82 ) 



temperature is determined exactly as described in Comm. N". 83. 

 The cryostat described could be relatively simple, because a vacuum- 

 glass of large dimensions was used. Excellent though a Aacuumglass 

 may be, it is still always to be feared that it bursts unexpectedly and 

 so damages the measuring apparatus. Indeed, one of the series ot 

 measurements was put an end to in this way. Hence, when mea- 

 suring apparatus are used to which we attach great value, because, 

 for instance, many other measurements have been made with them, 

 it is advisable when we w^ant to bring them in baths of constant 

 temperature below — 210° C, to use the cryostat described in 

 Comm. N". 83 III, where, though it is much more complicate than 

 the one described here, no vacuum glasses are required, and in this 

 the oxygen can be evaporated at a very low pressure, also with the 

 aid of the above mentioned large vacuumpump. 



Physics. — "]\fetho(h and apparatus used in the cryogenic lahoratory. 

 IX. The purifying of gases hy cooling combined ivith com- 

 pression, especially the preparing of imre hydrogen'' Com- 

 munication N" 94"^ from the Physical Laboratory at Leiden by 

 Prof. H. Kamerlingh Onnes. 



^ 1. To separate less volatile elements from a gaseous mixture 

 by cooling with liquid air belongs now to the ordinary operations 

 in laboratories. At Leiden it is applied on a fairly large scale to 

 reobtain ethylene in its pure form after it has been mixed with air. 

 In the experiments it repeatedly occurs that ethylene is contaminated 

 with air; from time to time when in the etl\ylene cycle of the cascade 

 process the condensation pressure of the ethylene increases, the gas 

 which remains behind after the greatest part of the gas used in the 

 cycle is liquefied, is blown otf and replaced by pure ethylene, in 

 order to reduce the condensation pressure to its ordinary amount. 

 All such mixtures and remnants with a larger or smaller pro[)ortion 

 of ethylene are collected in a large gasholder because of the expen- 

 siveness of this gas. The ethylene is afterwards frozen out from the 

 collected gas in a vessel cooled by liquid air. 



By cooling at a normal pressure we can from a mixture of gaseous 

 substances which differ very much in volatility, separate a large 

 portion of the least volatile substance when we go to temperatures 

 which though lying above the boiling i)oint of the one, reach far 

 below that of the other substance. How much of the impurity is 

 still in the remaining gas is then fairly well determined by the 



