( 756 ) 



compressed air, and c s for the cock which shuts off the level glass. 

 Of the compression tube, provided with the system of cocks, mercury- 

 reservoir and level-glass belonging to it a front-view is given in 

 fig. 3 of PI. I. The piezometer with the connections </ l and g 2 has 

 been represented in detail in tig. 2 of PI. I. 



The arrangement of the cryostat B which lias served for the deter- 

 minations mentioned in this communication, is described in Comm. 

 N°. 94". 



For the description of the apparatus serving for keeping the tem- 

 perature in the cryostal constant, we may also refer to this last 

 communication. Fig. 4 of PI. I may also serve for elucidating' this 

 description for the special case that our piezometer is placed in the 

 cryostat. 



The pressure is conveyed (see fig. 1, PI. I) from the compression- 

 tube to the manometer along c lx , c 8 , c ls , c 18 , c u and c ls . By closing 

 and opening c ]a the differential-manometer E 1 ) may be shunted in 

 and out. By means of die cocks c 8 and c„ it may be shut off from 

 the rest of the pressure-conduit, when greal differences in pressure 

 are brought about, or are to be feared. 2 ) 3 ). The apparatus are 

 placed in two rooms as has been indicated in the figure by a 

 dotted line. By closing one of the cocks c x% and c 14 the two parts 

 may be treated as independent systems. This was done when the 

 manometer was compared with the open manometer connected at c l7 . 

 The manometer C is the same as served for the investigations of 

 Comm. N°. 78. The reservoir D serves, if necessary, for eliminating 

 the injurious influence of small leaks, for which purpose it is placed 

 in ice. At c 18 it can be coupled to the system. In the experiments 

 of this communication there was no need to use it 4 ). The pressure 

 is exerted by compressed air, which enters through c, and c„ along 

 the drying tubes /'and G\ and is regulated by blowing off along c 84 . 

 The cocks c 18 , c 18 , c J0 , c tl , e aa and c 2S have analagous meaning to 

 c n , c 7 , c s , c 4 , c. and c t . 



1 ) This manometer, which was formerly used with the open standard-mano- 

 meter, (see G fig. 1 Comm. N°. 41) and had now been removed to the piezo- 

 meter, was of great use for finding leaks. 



2 ) A couple of mercury-receptacles, which served for receiving the mercury 

 that might overflow, have not been represented in the figure. 



3 ) The system which we have so far described and which belongs to the piezo- 

 meter, is placed in one of the rooms of the laboratory, situated in the immediate 

 neighbourhood of the cryogen department. The remaining apparatus which chiefly 

 belong to the manometer are erected in the room with the standard-manometer. 



*) The adjustment of trays of oil for the different couplings rendered the search 

 for leakages so easy, that an injurious leak needed never to remain. 



