( 10!? ) 



reference was then made as to be described later in anollier paper 

 dealing with that investigation. 



The measurements intended necessitated great accuracy in deter- 

 minations with the volumenometer. While in the researches described 

 in Comm. N°. 1J7 the volume of the gases whicli were admitted 

 froui the dilatometer to the volumenometer could be measured under 

 pretty high pressures (1 — ^ atm.), and consequently those pressures 

 had to be known with certainty only to 0.05 mm., it is desirable 

 in a determination of the compressibility of hydrogen at very low 

 temperatures ( — 259 to — 252°C.j, to measure pressures which may be 

 as low as 10 cm. to one five-thousandth of their value, so that in 

 this case an accuracy of 0.02 mm. is essential. The lowness of 

 the pressures to be measured necessitates a very high degree of 

 accuracy in the pressure measurements. My object in the present 

 investigation was to see in what way these high demands could be 

 satisfied, ') and to make sure that such was actually the case in the 

 results obtained. 



§ 2. Constants. I. Determinations of the Volumes. 



As has ah-eady been mentioned in Comm. N". 117 § 3, the 

 accurate volumenometer in use in the cryogenic laboratory -) 

 described in Comm. N". 84 was re-calibrated, from which at the same 

 time comparisons might be made with the data of Comms. N°. 88 

 and W. 92 and an idea of the accuracy to be reached could be 

 obtained. For this purpose a capillary tap with a fine drawn out 

 nozzle was fused to the lower end of the air-trap at Eb^ i^Pl. I. 

 Comm. No. 84). The calibrations were made with mercury with 

 which the apparatus was filled under high vacuum. During the pro- 

 gress of the calibrations the temperature of the volumenometer was 

 kept constant to within V4otli of a degree by means of the ther- 

 mostat described in § 5) '). 



1) A discussion of the degree of accuracy which was then reached was given by 

 Keesom in Comm. N\ 88 (1903). 



~) The volumenotneter with its auxiliary apparatus is shown on the left hand 

 side of PI. I Comm. W. 117 to whicli belongs fig. I PL I of Comm. N\ 84, 

 along with the modifying diagram shown on fig. 2 PI. I of this Communication. 

 In connection with fig. 2 PL L see also § 4 of this paper, l^'or the arrangement 

 of the volumenometer, scale, cathetometer, etc., see Comm. N". 117. For descrip- 

 tion of tlie tliermostat see § 5 of this Communication. 



3) Before proceeding to calibrate the apparatus it was thoroughly cleaned both 

 inside and out. In this process the screen Eqi (PL I, Comm, N'. 84) was replaced 

 by a new one, and the optical properties of the glass windows in the jacket were 

 investigated before the latter were replaced. 



