406 
attained in the measurements of KAMERLINGH ONNES and BRAAK Was 
about ¢ = 2.2, in our present investigation we were able to calculate 
the second virial coefficient 5 for the further region from ¢ = 0.7 
to about t= 0.5; by this means, since interpolation between t = 0.7 
and t—2.2 is not a matter of any difficulty, B becomes known 
over a very extensive region of reduced temperature (from t= 0.5 
to t > 12). The second reduced virial coefficient is therefore known 
for a single substance over a much more extensive region of tempe- 
rature than has hitherto ever been the case. This extension was 
especially to be desired as, in the first place, it allows a better 
comparison from the point of view of the law of corresponding states 
of B for hydrogen with its value for various other substances, 
and this will become of particular importance when the comparison 
can be extended so as to embrace monatomic substances (a commu- 
nication by KAMERLINGH ONNES and CROMMELIN will shortly appear 
dealing with the B for argon at low reduced temperatures). In the 
second place it allows us to put to the test theoretical deductions 
concerning / (for instance, the connection between the peculiarities 
of B with the peculiarities of the specific heats and of viscosity, and 
also of the dielectric constants and of penetrability by electrons). 
This is all the more important as B is related to that state which 
according to RemGanum can be called the planetary gas state in 
which, in allowing for the influence of collisions between moiecules, 
only two molecules need be considered, as the possibility of the 
proximity of others may be neglected. 
From B, moreover, one can calculate the experimentally deter- 
mined corrections of the hydrogen thermometer scale to the AVoGADRO- 
scale, which have hitherto been known only down to — 217° C., 
down to the lowest temperatures which can be measured with the 
hydrogen thermometer. (Cf. Comms. Nos. 1015 and 1020). 
The uncertainty in the adjustment of a cryostat bath to an accurate 
definite temperature and in the measurement of that temperature is 
much greater than that with which a temperature, once steadied, can be 
maintained constant. Since, now, uncertainty in the determination of 
the temperature is of great influence upon the values of 4 obtained 
from the observed pv4, it was decided to proceed with isothermal 
measurements so as to be as independent as possible of thermome- 
trical measurements. A further advantage of constancy of temperature 
in the comparison of values of pv4 at different pressures lies in the 
circumstance that the possible difference between the temperature 
of the gas in the piezometer and that of the thermometer in the 
bath is constant throughout. The advantages of isothermal measure- 
