296 
pra ='Aa + BA ds 
in which A gv 1.07258 ner ee fa 
Ba = 0.000667 
Again, on account of the small densities at which the volumeno- 
meter is used, the second of the terms on the right of the sign of 
equality plays but a small part in the result; it varies from 7.104 
of pva at density 1.1 to 1.10~4 of pv4 at density 0.15. The question 
to be investigated in the proposed test was if compressibility deter- 
minations with the volumenometer could give values of pv4 to within 
2 Ors 
As appears from the table at the end of $ 3 giving pvy as obtained 
from experiment and d4 as calculated, the accuracy attained in the 
compressibility determinations is as a rule somewhat greater than 
that which we desired. (Comm. No. 121, $ 1). To show more clearly 
the nature of the remaining deviations, values of 6,4 determined by 
formula (I) have also been calculated from the volumenometer results 
by themselves; in doing this, of course, a sufficiently good approxi- 
mation can be obtained only at the highest densities. 
§ 2. Summary of the experimental methods. To get as good an 
idea as possible of the reliability of the volumenometer determinations 
of compressibility at temperatures between — 252°C. and — 259° C. 
the compressibility was first measured at ordinary temperature within 
the same pressure limits as would be chosen or were to be expected 
at the lower temperatures. Measurements were made with two 
distinct quantities of distilled hydrogen. For the first series a pressure 
of half an atmosphere was chosen as the starting point, and it was 
desired to ascend to a pressure of 1.1 atm. while in the second 
series the limits chosen were 0.16 atm. to 9.5 atm. The apparatus 
was filled in the usual way (cf. Comm. No. 94/) after repeated 
evacuations and washings with hydrogen. 
For the determination at higher pressures measurements were 
made in the neck m, (see Plate I, Comm. No. 117) and pressures 
were obtained from the manometer @,--@, and the barometer 
6c—Op. In this an artificial constant pressure practically equal to 
the barometric pressure was maintained in the manner usually 
adopted in the Leiden Laboratory by means of the ice pot A. To 
eliminate changes due to temperature fluctuations the four menisci 
to be observed were read twice in reverse order. Measurements were 
then made in the necks m, and mm, (Pl. I loe. cit). To do this the 
tap 7, was closed, and, keeping &, closed, communication was esta- 
blished with a mercury pump through &,,, £,,. After careful evacu- 
