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still so large that the utmost was demanded of the dimensions of 

 the necessary vacuum glasses; which was of the more importance, 

 because the bursting of the vacuum glasses during the experiment 

 would not only be a most unpleasant incident, but might at the 

 same time annihilate the work of many months. 



Besides the difficulties given by the helium liquefactor itself, the 

 further arrangement of the cycle in which it was to be inserted, 

 offered many more. 



The gas was to be placed under high pressure by the compressor, 

 and was to be circulated with great rapidity. Every contamination 

 was to be avoided, and the spaces which were to be filled with 

 gas under high pressure were to have such a small capacity, that they 

 only held part of the available naturally restricted quantity of helium. 

 As compressor only Cailletet's modified compressor could be used, 

 a compressor with mercury piston, which had been arranged for 

 experiments with pure and costly gases, and was described in Com m. 

 N°. 14 (Dec. 1894) and Comm. N°. 54 (Jan. 1900), and which also 

 served for the compression of the helium in the expansion experi- 

 ments of last March (Comm. N°. 105) ! ). 



That it could only be charged to 100 atms., a fact which I had 

 sometimes considered as a drawback in the case of experiments with 

 helium, could no longer be deemed a drawback after the determina- 

 tions of isotherms had taught that even if the pressure of helium 

 compressed above 100 atms. at low temperatures is raised much, 

 the density of the gas increases but little. Accordingly I had not 

 gone beyond 100 atms. in my expansion experiments. The higher 

 pressures which Dewar and Olszewski applied in their expansion 

 experiments, have been a decided disadvantage, because they involved 

 the use of a narrower expansion tube. With regard to the circulation 

 now to be arranged, with estimation of the critical pressure at 7 

 or 5 atms. 2 ), according as b was put at a third or half that of hy- 

 drogen, a pressure of 100 atms. in the regenerator coil had to be 

 considered as sufficient according to the law of corresponding states. 

 But for a long time it was considered an insuperable difficulty 

 that the compressor conjugated to the auxiliary compressor could 

 circulate at the utmost 1400 litors of gas measured at the ordinary 



1 ) [Just as when it was used to get a permanent bath of liquid oxygen (com- 

 pleted 1894 Comm. N°. 14) it was now again in the pioneering cycle and rewarded 

 well the work spent on it, especially in 1888 when I was working at the problem 

 to pour off liquid oxygen in a vessel under atmospheric pressure by the help of 

 the ethylene cycle]. 



2 ) [The results of the isotherm of helium at — 259° to be treated in a following 

 communication were not yet available then; they point to a smaller valuej. 



