(157) 



The cooling of an apparatus of dimensions like the first of Linde 

 (weight J300 kilogrammes) by means of li(|ui(i air (oxygen) evapo- 

 rating in vacuo could not be thought of. And yet, according to 

 what has been said above, this had to be our starting point. 



It rather lay to hand to magnify the spiral (enclosed in a vacuum 

 glass) such as Dewar had used for his jet of hydrogen to solidify 

 oxygen, and so to get an apparatus with which air could be liquefied, 

 and which could then serve as a pattern for an apparatus to liquefy 

 hydrogen. It was indeed a similar construction with which in 1898 

 Dewar had statically liquefied hydrogen for the first time. About the 

 installation which apparently afterwards enabled Dewar to collect 

 large quantities of liquid hydrogen nothing further has come to my 

 knowledge. 



The arrangement of the Leiden hydrogen circulation is based on 

 Dewar's principle to place the regenerator spiral into a vacuum 

 glass (1896). As to the regenerator spiral itself Hampson's apparatus 

 for liquefying air (1896) has been followed because it appeared that 

 the proportions of this spiral have been chosen very favourably, and 

 with its small dimensions and small weight it is exceedingly fit, 

 according to the thesis mentioned above, to serve as a model for a 

 regenerator spiral to liquefy hydrogen of about — 205° at expansion 

 from a higher to the ordinary pressure. The other physicists, who 

 after Dewar have occupied themselves with liquid hydrogen, — 

 Travers 1900 and 1904, Olszewski 1902, 1904 and 1905 (the latter 

 rather with a view to obtain small quantities in a short time with 

 simple accessories) — have also built their apparatus after this model. 



The Leiden hydrogen liquefactor for constant use has enough 

 peculiar features to occupy a position of its own as an independent 

 construction by the side of the apparatus of Travers and Olszewski, 

 which do not satisfy the requirements for the Leiden measurements. 

 JMoreover I was the first to pronounce the principle according to which 

 this apparatus is built and from which follows that the regenerator 

 spiral fed with hydrogen that has been cooled by liquid oxygen (air) 

 evaporating at a given low pressure, must lead to the goal. 



The problem of making a circulation in order to maintain a bath 

 of liquid hydrogen — and of this problem the arrangement of the 

 liquefactor for constant use (which, tested with nitrogen, has really 

 proved efficient) is only a part — has not yet been treated by others. 



That also at Leiden we had to wait a long time for its solution 

 cannot be wondered at when we consider the high demands which, 

 I held, had to be satisfied by this cycle. For with a view to the 

 intended measurements I thought it necessary to pour a bath of 



