( 173 ) 



XII. Cryostat especially for temperatures from 

 — 252= to —259^ 



§ 1. The principle. In X § 1 I liave said that we succeeded in 

 pouring into the crjostat of Comm. N". ^-i'J- VIII a bath of liquid 

 hydrogen, maintaining it there and making measurements in it, but 

 then the vacuum glass cracked. By mere chance it hiippened 

 that the measuring apparatus which contained the work of several 

 series of measurements came fortli uninjured after removal of the 

 sherds and fragments of the vacuum glass. With the arrangement 

 which I am going to describe now we need not be afraid of an adversity 

 as was imminent then. Now the bath of liquid hydrogen is protected 

 against heat from outside by its own vapour. The new apparatus 

 reminds us in many respects of that which I used to obtain a bath 

 of liquid oxygen when the vacuum glasses were not yet known; 

 the case of the cryostat then used has even been sacrificed in 

 order to construct the apparatus described now. 



The principal cause of the cracking of vacuum glasses, which I 

 have pointed out in several communications as a danger for placing 

 precious pieces of apparatus into them are the great stresses 

 caused by the great differences in temperature between the inner 

 and the outer wall and which are added to the stresses which 

 exist already in consequence of the vacuum. To the influence of 

 those stresses it was to be ascribed, for instance, that only through 

 the insertion of a metal spring the vacuum tubes (described in Comm. 

 N". 85, April '05) could resist the cooling with liquid air. It some- 

 times happens that a vacuum flask used for liquid air cracks without 

 apparent cause and with the same cooling the wide vacuum cylinders 

 are still less trustworthy than the flasks. At the much stronger cooling 

 with liquid hydrogen the danger of cracking increases still. Habit 

 makes us inclined to forget dangers, yet we should rather wonder 

 that a glass as used for the cryostat of Comm. N". 94'^ VIII filled 

 with liquid hydrogen does not crack than that it does. 



In the new cryostat of PI. V the cause of the cracking of the vacuum 

 glass has been removed as much as possible and in case it should 

 break in spite of this we have prevented that the measuring apparatus 

 in the bath should be injured. The hydrogen is not poured directly 

 into the vacuum glass B\^ but into a glass beaker Ba, placed in the 

 vacuum glass (comp. Comm. N°. 23, Jan. '96 at the end of § 4) but 

 separated from it by a new-silver case, which forms, as it were, 

 a lining (see X, L PI. I). P'urther the evaporated hydrogen is led 

 along the outer wall of the vacuum glass B\^. To be able to work 



