( 108 ) 
reliability of the values, which can be derived from them for VAN 
DER WAALS’ quantities «a and 5 made me resolve to contribute to 
the investigation of the isothermal of hydrogen, which has for many 
years been under consideration in the Physical Laboratory at Leiden 
(comp. Comm. N° 14 by KAMERLINGH Onnes, Proceedings of 
December 29% ’94) and to do this by again experimenting upon 
this gas at the ordinary temperature. 
The apparatus, methods and investigations discussed in the Pro- 
ceedings of Oct. 29th ’98, June 24th ’99, Dec. 29th ’00, Jan. 26% 
01 and May 25% ’01 (Comm. Nes, 44, 50, 67 and 70) allow us 
to make such a precise determination of pressure and volume that 
we can a priori count upon the possibility of determining the values 
to be ascribed to a and b by observations at pressures up to 60 atm. 
The hydrogen has been prepared as described in Comm. N°. 27, § 5 
(Proceedings of May 30% ’96) and N°. 60 § 22 (Proceedings of 
June 30% ’00). The four piezometer tubes were simultaneously filled 
at the apparatus, and were six times entirely exhausted by means of 
the mercury pump and heat, and then filled again. 
§ 2. The normal volume. To calculate each isothermal deter- 
mined by means of a piezometer it is of the highest importance to 
know the normal volume. It seems that in most of the measurements 
of other observers its determination has left much to be desired; 
but KAMERLINGH ONNES’ arrangement of the piezometer tubes allows 
us, as will be seen from this section, to attain the degree of accuracy 
desired. 
Before being placed into the compression apparatus the piezometer 
tubes were brought into the water bath (represented Comm. N° 50, 
Pl. 2, fig. 5) between the two brass walls of which water at 20° C. 
from the thermostate flowed during continual stirring. The inner 
copper vessel was closed at the upper end by means of an india 
rubber stopper, through which a thermometer had been passed and 
also an air tight connecting tube!) to the barometer, which tube 
1) This connecting tube could not easily be made at the lower end, as the inner 
copper vessel was standing loose on three corks, and a long tube reaching from the 
stopper to the bottom could not be used, as round the steel flanged tube cemented 
on the glass tube only half a centimeter space was left. (In order to prevent during 
the experiments the leakage of mercury, the hard red cement of MENDELEJEFF at the 
lower end was replaced by soft black cement, which moreover was covered at the 
jnside with an india rubber solution). In order to ascertain, that this connecting 
tube did not interfere with the attainment of equilibrium of pressure, the measurement 
