( 182 ) 



quickly enough established with the large vacuumpump, which exhausts 

 to 2 mm., so this will have to be investigated on another occasion. 

 The deficient connection, however, has certainly mule the pressure 

 decrease below 1 cm., and perhaps even lower. That 7 mm. has 

 been reached, is not unlikely. 



At 9.40 only a few cm 8 , of liquid helium were left. Then the 

 work was stopped. Not only had the apparatus been taxed to the 

 uttermost during this experiment and its preparation, but the utmost 

 had also been demanded from my assistants. 



But for their perseverance and their ardent devotion every item 

 of the program would never have been attended to with such perfect 

 accuracy as was necessary to render this attack on helium successful. 



In particular I wish to express my great indebtedness to Mr. G. J. 

 FliM, who not only assisted me as chief of the technical department 

 of the cryogenic laboratory in leading the operations, but has also 

 superintended the construction of the apparatus according to my 

 direction, and rendered me the most intelligent help in. both respects. 



§ 5. Control experiments. All the gas that had been used in 

 the experiment, was collected in three separate quantities and com- 

 pressed in cylindres Quantity A contains what was finally left in 

 the apparatus. Quantity B has been formed by evaporation of a 

 certain quantity of liquid helium. Quantity C is the remaining part 

 that has been in circulation. Together they yielded the same 

 quantity as we started with. They were all three exploded with 

 addition of oxyhydrogen gas and excess of oxygen ; no hydrogen could 

 be demonstrated. For the density (in a single determination) we 

 found (0=16) .4 = 2.04, 73=1.99, C==2,02). 



The spectrum of the pas used for the experiment put in a tube 

 with mercury closure without electrodes and freed beforehand from 

 vapour of water and fat at the temperature of liquid air, answered 

 (only the spectrum of the capillary has been investigated) the de- 

 scription given by Collie of the spectrum of helium with a trace of 

 hydrogen and mercury vapour. 



Spcctroscopically both the distilled C, and B were somewhat 

 purer than the original gas. In the latter the hydrogen lines gained 

 in case of high vacua, in the former the helium disappeared last. 

 The hydrogen, from which the latter has still been cleared, must 

 be found in A. By means of absorption by charcoal 8 cm 8 , of hy- 

 drogen was separated from this. To this would correspond a difference 

 in percentage of hydrogen before and after the experiment of 

 0.004 •/,. 



