1060 
2. Apparatus and method. The measurements were carried out 
with a vapour-pressure apparatus which will be described in a 
future paper on the vapour-pressures of neon and hydrogen. The 
small bulb in which the gas is liquefied is shown at A in fig.1 of 
the preceding communication. It is placed in the experimental chamber 
É of the hydrogen-vapour cryostat half way between bottom and 
top side by side with a helium-thermometer 7h, and a resistance- 
thermometer 2. The vapour-pressure apparatus is so arranged, that 
the quantities of gas which were liquefied at a given temperature 
between the beginning and the end of condensation could be measured. 
Using the values obtained in that manner at different temperatures 
in the neighbourhood of the critical temperature it was possible in 
connection with temperature and pressure by means of an extra- 
polation over a small range to derive the critical temperature and 
pressure within the limits of accuracy given above. 
The value to be ascribed to the critical pressure can be checked 
by means of the pressure at the point of inflexion of an isothermal 
immediately above the critical temperature, which was determined 
specially for this purpose. 
The manner in which the extrapolation was carried out will be 
elucidated by means of a diagram in the next communication dealing 
with the critical point of hydrogen. 
We mention in this connection that owing to the impurity of the 
neon referred to, small though it was, the heterogeneous isothermals 
in a pressure-density diagram did not run exactly parallel to the 
density-axis, whereas they did with hydrogen. 
Owing to these pressure-differences along the heterogeneous 
isothermal it was more difficult than in the case of hydrogen to 
arrive at an exact calculation of the critical constants. 
As regards the preparation of neon it may be mentioned that the 
impure gas forming our stock was first freed from hydrogen after the 
addition of oxygen by explosion, it was then frozen a number of 
times at the air pump and ultimately repeatedly distilled over carbon 
cooled in liquid air. Although often repeated and carefully carried 
out these operations have evidently not been sufficient to free the 
neon completely from admixtures. 
The pressure measurements were made by the aid of the closed 
hydrogen-manometer 4/,, which has been often mentioned in previous 
communications of this series (see for instance Comm. N°. 146c). The 
temperatures were measured with the constant-volume helium gas- 
thermometer 7h, referred to above; the bulb had a volume of 
110 ce, the “waste space’ was .7 °/, of the volume of the bulb; 
