( 949 ) 
for this purpose is given by the difference of level between the 
mercury in the volumenometer and that in the manometer tube plus 
the barometric height when, as was the case in all our experiments, 
this manometer tube is in connection with the constant pressure 
reservoir FR. The height of the barometer was given by Bar. (For 
this method see Comm. No. 60, Sept. 1900, and Comm. No. 84). 
As a rule the tap 4 
tained between M/ and /’; it was kept closed while readings were 
being made. Sometimes, when one wanted better adjustment or 
readings, advantage was taken of the tap 4, to make the pressure 
‚a Was opened while equilibrium was being ob- 
in FR a few centimeters higher or lower than atmospheric. 
§ 4. Erperimental method. Auxiliary apparatus. A tap k, connects 
the volumenometer with a rigidly constructed 7-piece which through 
other taps connects the volumenometer and the dilatometer with each 
other and with the apparatus in which the oxygen is stored under 
high pressure; these are a cylindrical reservoir P, and an auxiliary 
compressor A in the glass tube of which marked A, the oxygen is 
contained over mercury. All parts of the apparatus can be evacuated 
along &,,, k,, 4, and this is of course always done, and &,, and c, 
kept closed before any measurements are made and before any 
oxygen is admitted to the tube A, from the reservoir P,. When, as 
is almost always the case at very low temperatures, the pressure 
in the reservoir P, is higher than the maximum vapour pressure of 
oxygen at the temperature ef the dilatometer, the oxygen can be 
simply distilled over into the dilatometer from the reservoir P,, and 
by regulating the taps, the liquid can be brought to the desired level 
in the graduated stem of the dilatometer. The auxiliary compressor 
is brought into use in making the adjustments when the pressure 
in the reservoir is lower than that of the liquid oxygen in the 
dilatometer. The auxiliary compressor, indeed, by closing #,, 4, and 
opening k,, k,, k, may be coupled to the dilatometer d in Cr to 
form a piezometer such as is used for the determination of isotherms 
(see Comms. No. 97a, Pl. I and No. 69 PI. I and ID): oxygen is 
then admitted to the reservoir along 4, 4,, 4, and is then trans- 
ferred to the dilatometer d by forcing mercury into the reservoir by 
means of compressed air (compare the Plate quoted from Comm. 
No. 97a with that given with the present paper; the letters used are 
the. same in the two cases and their meaning will be found in 
that Communication); the oxygen is then brought to the desired 
height in the dilatometer by the admission or removal of oxygen. 
At the same time one can bring the mercury in A to the graduated 
9 
