755 



appeared that there are discrepancies between some of Amagat's 

 results inter se, as well as between Ama(jat'k observalions on hydrogen 

 at high pressures on one side, anrl Sciialkwuk's very accurate 

 observations for low pressures on the otiier side ^). 



For all these reasons it seemed desirable to construct an apparatus 

 with which gas-isotherms might be measured up to the highest 

 attainable pressures. And as it is self-evident that the cost of sucli 

 a set of apparatus could not be defrayed from the ordinary means 

 of a laboratory, the board of the van der WAALS-fund resolved 

 ab-eady in 1904 to grant money for tiiis purpose. It is owing 

 to the strong support given by the van vvai WAALs-fund all these 

 years that we are now able to communicate the first results. Our 

 cordial thanks are due to the board of the van der WAALS-fund, 

 and further to all who helped to support the fund. 



Tn the following pages we shall of course not give an account of 

 all the difficulties that confronted us, and the way in which they 



were finally surmounted. We shall confine 

 ourselves to a description of the arrangement 

 in its present form, and only mention in a 

 few words now and then what considerations 

 have led to this final form. We shall suc- 

 cessively discuss the measurement of the 

 pressure, the volume, and the temperature. 



B 



C-^ 



A 



SIZ 



\/ 



A. Measurement of the Pressure. 



The measurement of the pressure in abso- 



. . lute measure takes place by means of SciiAFFER 



TTJ - hr i and Budenberg's pressure balance. In principle 



^^ " this apparatus consists of a steel piece A 



(fig. 1} with cyiindric boring, which at 



about half the height passes into a wider 



cyiindric boring. A differential piston B 



fits in this boring, which piston is ground 



into the two cylindres with the utmost care. 



By means of a side tube the cavity C can 



be connected with the space where the 



pressure is to be measured. This side tube 



and the space under the piston are filled 



with machine oil. By means of a mould, 



1) Brinkman, Thesis for the Doctorate, Amslerdam p. 34. 

 Schalkwijk. Thesis for llie Doctorale, Leiden 1908, p. 120 et. seq. 

 Keesom. Thesis for the Doctorate, 1904, p. 57. 



Fig. 1. 



