m 



Thns for every platinum contact in the measuring tube the pressure 

 at the corresponding' volume is determined. Then the pi-essure is 

 diminished, so (hat two series of observations of the same results 

 must be obtained, but one passed Ihrough at increasing, the other 

 at decreasing pressure. The pressure differences at two corresponding 

 observations of two such series rarely amounted to more than 50 

 grams. It must of course be continually verified whether the tem- 

 perature differences inside and outride the measuring tube have 

 disappeared. This has taken place when the resistance of the teu)- 

 perature wire (cf. Comm. 6 p. 833) has become constant. 



In the measurement of the highest pressures, so when the large 

 "head" of the pressure balance is used, i.e. between 1200 — 2400 

 atmospheres, the accuracy of the pressure measurement becomes less, 

 especially on account of the increasing viscosity of the mineral oil 

 used as transmission liquid. Yet the error will certainly remain 

 below 7,„„o. 



§ 4. Determinations hetioeen 2 and 200 atias. 



As was already mentioned in Comm. 6 (p. 830) unforeseen diffi- 

 culties prevented us from determining the normal volume of the 

 quantity used in the apparatus itself'. For the determination of the 

 isotherm of 20^ C. we could make use for the calculation of the 

 normal volume of Schalkwijk's measurements, as will become clear 

 from the discussion of our results. Such data were wanting for 15°. 5 C 

 And in order to be able to carry out all the same an accurate 

 comparison with Amagat's measurements, we have executed measure- 

 ments at lower pressures at that very temperature. In this way a 

 control was obtained whether the equation for low pressures derived 

 from the measurements may be extrapolated. 



We shall return to these points when our results are discussed, 

 and, first give a desci-iption here of these measurements too. As we 

 again wished to use a large quantity of gas, the iron vessel D used 

 in the large apparatus was used as a pressure cylinder. We then 

 could fill the piezometer with a quantil} of gas of the order of 

 magnitude of 1 liter at 1 atmosphere. 



The piezometer originally consists of two pieces. The upper part 

 was as the upper reservoir of our ordinary measuring tubes. At the top 

 at a (fig. 13) there are 4 etched lines to be used after the cleaning 

 of the tube. Lower down (here is a widening h; under this a sealed 

 in platinum wire c, and at last an etched scalar division d. This 

 tube was connected with a capillary, and bent round. A platinum 

 wire is sealed into the bent part. Here a current can enter, and 



