Atl 
Neither are the numerous thermometers shown in the diagram which 
were suspended along the whole apparatus. 
Finally, the connections Zk,, Zh, lead to the hydrogen reser- 
voir Z,. To this reservoir is attached a side tube with a valve, 
Zk,, through which the whole apparatus can be filled with hydrogen; 
it is also provided with a purifying chamber Zy, through which the 
gas passes on its way to the measuring apparatus and consisting of 
a tube filled with glass wool surrounded by a Dewar flask containing 
liquid air. After the measurements the gas can be collected in Z, 
through Zh, Zk,,. 
We may refer to Comms. Nos. 83, 94c, 94d, 94e, and 121a for 
descriptions of the thermostat, the water circulation and the cryogenic 
bath and auxiliary apparatus. 
§ 3. The hydrogen. The apparatus was filled with distilled hydrogen 
by means of the arrangement described in Comm. N°. 94e, § 2; the 
tap Zk,(see fig. 1) was utilised for the repeated evacuations and washings 
with hydrogen. 
§ 4. The temperatures. The thermostat supplied the water circu- 
lation, W, with water at very uniform temperature. (See Comm. 
Nt 21a). 
Stirring was continuous during the measurements. A thermometer 
divided into 20-ths of a degree and calibrated by the Reichsanstalt 
was attached to the stirrer of the voiumenometer, the mean tempe- 
rature being thus obtained. The influence was studied beforehand of 
fluctuations in the room temperature upon that of the water in the 
jacket surrounding the volumenometer, and it was found sufficient 
to keep it constant to within one deg. Cent. This was always done. 
(For further details see dissertation by W. J. pr Haas). Every deter- 
mination of the volumenometer temperature can then be regarded 
as certain to within 0°.02 C. 
The temperature of the cryostat was regulated in the usual way ; 
great care was devoted to keeping it constant by Mr. G. Horst, 
whom we wish to thank for his assistance. It would take up too 
much space here to give all the curves of this temperature regulation, 
but as an example we may state that in the determination of the 
isotherm at — 255°.5 C., made on the 24' of June, and on the 8 and 
14 of July, 1911, the values of the differences at five points from 
the first determination were 
0.005, 0.012, 0.010, 0.000 degrees Centigrade. 
This corresponds to an uncertainty of 0.00004 in the value of pv . 
ANS 
