( 179 ) 



produced by the regenerative cascade. Enough but not too much, 

 because for operations with liquid hj^drogen (comp, X) and also for 

 other experimentations in the realm of cryogenic work it is very 

 important that we should dispose of such a relatively abundant 

 stock of liquid air as is produced by the Leiden cascade. 



XIV. Preparation of jmre hydrogen through distillation 

 of less pure hydrogen. 



It was obvious that we could obtain pure hydrogen for the 

 replenishment of the thermometers and piezometers ^) when we distil 

 liquid hydrogen at reduced pressure ''), and then evaporate the very 

 pure liquid thus obtained. Therefore the following apparatus has been 

 constructed (fig. 5). 



A vacuum glass A is connected with the liquefactor (see PI. I 

 and III at N^) or with a storage bottle, exhausted and filled with 

 liquid hydrogen as indicated in X §> 7. Then C (exhausted beforehand) 

 in the vacuum glass B is filled several times out of A, and the 

 vacuum glass B is connected with B^^ to the liquefactor and exhausted 

 like A and also filled with liquid hydrogen and connected with the 

 ordinary airpump at B^ so that the hydrogen boils in B atGOm.m. 

 Then hydrogen is distilled over along c^ into the reservoir C, we 



1) In Comm. N". 94e (June '05) I have mentioned that a purification through 

 compression combined with cooling might be useful in the case of hydrogen even 

 after the latter in the generating apparatus (Comm. N^. 27, May '96 and N*^. 60, 

 Sept. 1900) had been led over phosphorous pentoxide. I said so especially with a 

 view to the absorption of water vapour as, with due working, the gas — at least 

 to an appreciable vapour tension — cannot contain anything but HoO and SO4H2. 

 How completely the water vapour can be freed in this manner appears from a 

 calculation of Dr. W. H. Keesom, for which he m.ade use of the formula of Scheel 

 (Verb. D. phys. Ges. 7, p. 391, 1905) and from which follows for the pressure of 

 water vapour (above ice) at — 180°G. 10— i® mm., so that water is entirely held back 

 if the gas remains long enough in the apparatus. This holds for all substances of which 

 the boiling point is higher than that of water (SOg vapours, grease-vapours etc.). 

 The operation is therefore also desirable to keep back these substances. As to a gas 

 which is mixed only with water there will remain, when it is led in a stream of 3 liters 

 per hour through a tube of 2 cm. in diameter and 8 cm. in length over phosphorous 

 pentoxide, no more than 1 m.gr. impurity per 40000 Hters (Morlev, Amer. Journ. 

 of So. (3) 34 p. 149, 1887). This quantity of 1 m.gr. is probably only for a 

 small part water (Morley, Journ. de chim. phys. 3, p. 241, 1905). Therefore the 

 operation mentioned would not be absolutely necessary at least with regard to 

 water vapour when a sulïicient contact with the phosphorous pentoxide were 

 ensured. But in this way the uncertainty, which remains on this point, is removed. 



2) This application follows obviously from what has been suggested by Dewar, 

 Proc. Chem. Soc. 15, p. 71, 1899, 



