( 175 ) 



the experiment in the designed way would be unadvisable, but how 

 near this limit was has appeared later. 



In all these considerations the question remained whether everything 

 that could appear during the experiment, had been sufficiently taken 

 into account in the preparation. So we were very glad when the 

 calculation of the last determined points on the isotherm of — 259° 

 shortly before the experiment confirmed that the ttoYi.Y.-point [though 

 below the boiling point of hydrogen] lay somewhat above this lowest 

 temperature of preliminary cooling, and at least the foundation of the 

 experiment was correct. 



In the execution I have availed myself of different means which 

 Dewar has taught us to use. I have set forth the great importance 

 of his work in the region of low temperatures in general elsewhere 

 (Comm. Suppl. N°. 9, Febr. 1904), here, however, I gladly avail 

 myself of the opportunity of pointing out that his ingenious dis- 

 coveries, the use of silvered vacuum glasses, the liquefaction of 

 hydrogen, the absorption of gases in charcoal at low temperatures, 

 together with the theory of van der Waals, have had an important 

 share in the liquefaction of helium. 



§ 2. Description of the apparatus. The whole of the arrangement 

 has been represented on PI. I. We mentioned before that in virtue 

 of the principles set forth in Comm. N°. 23 the construction of the 

 helium liquefactor (see PI. II and III) was as much as possible an 

 imitation of the model of the hydrogen liquefactor described before 

 (Comm. N°. 94''', May 1906), to which I therefore refer in the first place. 



It was particularly difficult to keep the hydrogen, which eva- 

 porating under a pressure of 6 cm. is to cool the compressed 

 helium to 15° K. (just above the melting point of hydrogen), on the 

 right level in the refrigerator intended for this purpose. This diffi- 

 culty was surmounted in the following way. The liquid hydrogen is 

 not immediately conveyed from the store bottles into the refrigerator, 

 but first into a graduated glass Ga in the way indicated before, 

 which on comparison of the figures from Comm. N°. 94/ and N°. 103 

 PI. 1 fig. 4 does not require a further explanation. This graduated 

 glass was a non-silvered vacuum glass, standing in a silvered vacuum 

 glass Gb with liquid air, in which on cither side the silver coating 

 had been removed over a vertical strip so as to enable us to watch 

 the level of the hydrogen in the graduated glass. From this vacuum 

 glass the liquid hydrogen is siphoned over into the hydrogen refri- 

 gerator by means of a regulating cock P. To see whether the level 

 of the liquid in the refrigerator takes up the right, position, the 

 german silver reservoir N x of a helium thermometer has been 



