( 755 ) 



the net of isotherms are only few in number, but these few points 

 have been determined with particular care, so that, so to say, they 

 form normal places in the examined region of the equation of state, 

 with which without preliminary adjustment we may set about the 

 calculation of individual virial coefficients. Characteristic of them 

 is that every group of such normal places obtained by deter- 

 minations with the piezometer and manometer (see Comm. N os . 69 

 and 78) lies really on the same isotherm (that of about — 104°, 

 — 136°, —183°, —195°, —205°, -213° and —217°), and that on 

 these same isotherms every time a point has been obtained at small 

 density by a determination with the hydrogen thermometer (see 

 Comm. N°. 95* Oct, 1906). The great difficulty x ) of this investigation 

 lies in obtaining the required constancy and stability of the low 

 temperatures. Accordingly the arrangement of reliable cryostats was 

 made a separate subject of investigation at Leiden, (cf. Comm. 

 Ne*. 83 and 94). 



This investigation comprises three series of piezometer-determina- 

 tions at densities respectively about 70, 160, and 300 times the 

 normal. 2 ) Several of the observations mentioned here lie in the 

 neighbourhood of the curve of the minima of p v. They enable us 

 to determine the shape of this curve pretty accurately (see § 13). 



We confine ourselves in this communication to our observations 

 themselves. A discussion of them, also in connection with the results 

 of other observers, will be given in a following communication. 



§ 2. Survey of the apparatus used. 



a. On PI. I in fig. 1 we find a schematic 3 ) representation of the 

 system of the apparatus for measurements and auxiliary arrangements, 

 excepted those which serve for keeping constant the temperature 

 in the cryostat. The compression apparatus A is the same as that 

 mentioned in Comm. N°. 84 (March '03). For the meaning of the 

 system of tubes, cocks and other parts we may refer to Comm. 

 N°. 69 and N°. 84. The same figures have been used, except that 

 in this communication c, is used for the cock which admits the 



*) Witkowski, whose important investigation on the expansion of hydrogen 

 (Bulletin de 1' Académie des Sciences de Cracovie 1905) had already appeared 

 before the experiments mentioned in this communication had been completed, 

 already mentions this as an explanation for the fact, that he has dropped the 

 direct determination of isotherms at temperatures lower than — 147°. 



2 ) The limits are chiefly given by the pressure under which the gas stands ; 

 they are about 20 and 60 atms. 



3 ) The individual apparatus arc represented on the same scale, the connections 

 schematically. 



