( 950 ) 
part of the stem A, by allowing oxygen to return to P, if necessary. 
The auxiliary compressor is evacuated before the admission of mer- 
cury ; if may also be used in conjunction with a metal manometer 
previously calibrated with the open manometer of Comm. N°. 44 or 
with the closed manometer of Comm. N*. 78 *) and with the pres- 
sure gauge and scale C (see Pl. 1) for reading off the vapour pressure, 
of the liquid oxygen in the dilatometer. This measurement is made 
while the meniscus is in the middle of the dilatometer reservoir dj, 
and care is taken to see that equilibrium, as shown by the stationary 
condition of the meniseus in A, is obtained at the same pressure, 
when there remains but a very small quantity of liquid in the 
dilatometer ; from this it is seen that the temperature is the same 
over the whole dilatometer, and in particular that it is the same 
for the appendix and for the middle of the bath where the tempe- 
rature is measured. 
The copper cylindrical reservoir P, with the tap /,, is of the type 
that is commonly used in the cryogenic laboratory; to it is attached 
a small manometer, the pressure on which shows the quantity of gas 
that is still left in the reservoir. The oxygen that has been used for 
the measurements can be brought back again by distillation on 
immersing the reservoir in liquid air. The space between /:, and 4:5 
is used for taking definite small quantities of gas from P, . 
For a detailed description of the arrangement of the cryostat Cr 
(Pl. 1 of this Communication with the same letters as in Comm. N°. 974) 
and for particulars regarding the cycle of liquid gas that is used 
when the bath that is being worked with is liquid oxygen boiling 
under reduced pressure, we may refer to Plate I of Comm. N°. 97a 
(March 1907) and to Comm. N°. 94d (June 1905). For the cases in 
which the temperatures are given by baths of liquid methane and 
liquid ethylene the apparatus is the same in principle. The place 
occupied by the piezometer in the researches to which reference has 
just been made is now taken by the dilatometer d. Instead of a 
single resistance thermometer as was used in Comm. N°. 94d we 
now used two platinum resistance thermometers*) each of them with 
four leads and of the type constructed wholly of platinum and glass 
(see Comm. N°. 995 § 2); the glass cylinder was heated until it was 
beginning to become slightly soft and then the very fine platinum 
1) We wish to record our thanks to Dr. G. Dorsman and Mr. G. Horst for the 
care with which they executed this work. 
2) The resistance thermometers were calibrated and the temperature determinations 
were made by Mrs. pe Haas—Lorentz: we gratefully acknowledge the careful 
assistance she gave us in this. 
