708 



is the pressure at C (fig. 1). The pressure at (he phice wliei-e one 

 wants to know it, in casn at the place where the isotherm measure- 

 ments talte place, must be derived from that in C by means of a 

 correction for the hydrostatic pressure dilference in C and the first- 

 mentioned place. 



The measurement of the pressures above J 250 kg. takes place in 

 exactly the same way, only (hen the ''head" A is exchanged for another 

 cylindre and piston whose effective area is only 7^ cm."' instead of 1 q\\\\ 

 Wiih this "head" therefore pressures of 5000 Kg. per cm" might bo 

 attained. The firm Sch\ffer and BroENBERG, however, informed us 

 in 1906 when they prepared the apparatus, that already at about 

 4000 atm. a permanent change of form of the cylindre A was to 

 be feared in consequence of a transgression of the limit of elasticity 

 of the steel, so that this pressure could not be exceeded for the 

 pressure balance. Since then Bridgman has succeeded in far exceeding 

 this limit of pressure by means of apparatus of newer kinds of steel. 

 The question, however, remains ' whether his apparatus could be 

 modified for the determination of gas isotherms. Apart from the 

 much greater complexity and dimension of tiie apparatus also the 

 question of a transmission liquid which could be used in the absohite 

 pressure gauge, is to be considered. Tlie machine oils, which we 

 always used as transmission liquid in the following investigations, 

 because they best remove the fricfiou between the cylindre and piston, 

 begin to be so viscous already at room temperature and 3000 atm., 

 that the pressure gauge begins to be slow in its indications, and 

 also the transmission of the pressure in the narrow channels becomes 

 highly uncertain. For this reason we have for the present confined 

 ourselves to pressures below 3000 atm. 



The very great value of the \iscosity of the mineral oils at high 

 pressures is ascribed by Tammann and Bridgman to the solidification 

 of these substances. With the apparatus, however, described in N". 4 

 of these communications (These Proc. XV p. 1021) nothing is to 

 be perceived of a deposition of solid substance at these pressures. 

 The oil remains as transparent v/hen this pressure is ai)proache(l as 

 it was at first, nor is anything to be oliserved of crystallisation. We 

 have, therefore, only to do with a very viscous fiuid, possibly a 

 continuous transition into an amorphous solid phase. 



B. The volume measurement. 



For the measurement of volume we have made use of a some- 

 what modified method of the electric contacts. Just as with Amagat, 



