( 763 ) 



As far as the pressure permitted, three points were chosen in 

 every series on every isotherm for the determinations of the isotherms 



ascending with about equal differences of density, which offers 

 advantages for the calculation of the virial coefficients (see § 12). 

 The readings were adjusted at these points by bringing the mercury 

 at the bottom, in the middle and at the top of the divided stem b 4 . 

 By way of control the two points in the middle and at the bottom 

 of the stem were for some determinations determined once more 

 with decreasing pressure. 



For every determination we waited till both the temperature and 

 ■the pressure were constant, and we could assume that the equilibrium 

 of temperature and pressure had been established. This will be the 

 case when the meniscus in the divided stem is moving up and 

 down within the same narrow limits. The stability of the tem- 

 perature was ensured by a good regulation, and that of the pressure 

 was easily obtained and preserved by paying attention to the oil- 

 trays mentioned in § 2, which immediately betrayed the slightest 

 leakage l ). 



When the above mentioned constant state had set in, some readings 

 of the piezometer and the manometer were alternately made. If they 

 agreed, we proceeded to the next point. 



With regard to the regulation of the temperature the measure- 

 ments took place under the same circumstances as the investigations 

 with the hydrogen thermometer described in Comm. N°. 95 e (Oct. 

 1906). Besides the resistance-thermometer for regulation and deter- 

 mination of the temperature the thermo-element was also used here 

 by way of control for the determination of the temperature. The 

 indications of the resistance-fhermometer, however, proving more 

 reliable than those of the thermo-element, only those of the latter 

 apparatus were used for the calculations. All possible care was 

 always taken that the temperatures at which determinations were 

 made, lay as close as possible to those which have been used for 

 the calibration of the resistance-thermometer to render the corrections 

 small, and the accuracy of the determination of the temperature as 

 great as possible. 



The regulation of the pressure took place according to the indication 

 of the metal manometer .1/ of PI. I, tig. 1. If we passed but slowly 

 from one pressure to the other the thermal process in the reservoir 

 which attended it, was so slight, that the regulation and the measure- 



l ) Formerly this often required a long and sometimes fruitless search (cf. e.g. 

 comm. N°. 70 p. 8). 



52 

 Proceedings Royal Acad. Amsterdam. Vol. IX. 



