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ruination of' the isotherms could decide how helium could be made 

 liquid. Hence we had proceeded with what might conduce to making 

 a favourable result for the critical temperature at once serviceable. 

 Thus the preparation of' a regenerator coil with expansion cock 

 in vacuum glass (to be used at all events below the point of inver- 

 sion), and the preparation of' pure helium was continued. Of the 

 latter a quantity had even been gradually collected sufficiently large 

 to render a determination of the Joule-Kelvin effect in an apparatus 

 already put to the test in prelimininary investigations possible, and 

 to enable us to make efficient expansion experiments. 



All at once all these preparations proved of the greatest importance 

 when last year (Comm. N'. 102a) the isotherms began to indicate 

 5° K to 6° K for the critical temperature, an amount which according 

 to later calculations, which will be treated in a subsequent paper, 

 might have been put slightly higher (e. g. 0,5 f ), and which was 

 in harmony with the considerable increase of the absorption of helium 

 by charcoal at hydrogen temperatures, on the strength of which Dewar 

 had estimated the critical temperature of helium at 8° K. For 

 according to the above theorem it was no longer to be considered 

 as impossible to make helium liquid by means of a regenerator coil, 

 though this was at variance with the last experiments of Olszewski, 

 who put the critical temperature below 2°. 



It is true that the conclusions drawn from the isotherms left room 

 for doubt. It seemed to me that the isotherms at the lowest tempe- 

 rature yielded a lower critical temperature than followed from the 

 isotherms at the higher temperatures, which is due to peculiarities, 

 which have been afterwards confirmed by the determination of new 

 points on the isotherms. So there was ample room for fear that 

 helium should deviate from the law of the corresponding states, and 

 that still lower isotherms than those already determined should give 

 a still lower critical temperature than 5° K., and according as the 

 critical temperature passed on to lower temperatures the chance to make 

 helium liquid by means of the Joule-Kelvin effect beginning at the lowest 

 temperatures to be reached with liquid hydrogen (solid hydrogen 

 brings new complications with it) became less. This fear could not 

 be removed by the expansion experiment which I made some months 

 ago, and in which I had thought I perceived a slight liquid mist. 

 [Coram. N°. 105 Postscriptum March 1908). For in the first place 

 only an investigation made expressly for the purpose could decide 

 whether the mist was distinct enough, and whether the traces of 

 hydrogen the presence of which was still to be demonstrated spec- 

 troscopically, were slight enough to allow us to attach any im- 



