( 941 ) 
on the equation of state and its graphical treatment in the Eney- 
klopädie der Mathematischen Wissenschaften. 
One of the most important of these reduced magnitudes seems to 
be the rectilinear diameter. In 1899) it was shown that the law 
of corresponding states was not true as regards the diameter for 
substances as a whole. On the supposition that the diameter is also 
a straight line for substances of very low critical temperature, the 
measurements made by Dewar and by Wrosiewski show that for 
these substances the reduced slope differs greatly from that for ordi- 
navy normal substances. This result led to the expectation that the 
investigation of the diameter would reveal a characteristic feature 
of the whole representation of the differences between the equations 
of state for substances of low critical temperature and those for 
ordinary normal substances, and this the more so as VAN DER WAALS’ 
recent researches upon molecular conglomerations in the liquid state 
have brought the rectilinear diameter to the front as a means of 
determining the law governing such conglomerations. ; 
In an investigation of the diameter of substances of low critical 
temperature the first question encountered is the following. It has 
been shown that for. ordinary normal substances the diameter is 
straight to a high degree of approximation: is this also the case for 
substances of low critical temperature? Does the deformation of the 
reduced surface, while occasioning a change in the orientation of the 
diameter, leave unaltered its rectilinear nature as ought to be the 
‘ase if this straightness were intimately connected with the innate 
characteristics of the liquid state? This is the fundamental problem 
that we have attacked, and it is answered in the affirmative by the 
measurements on oxygen which we publish in the present paper *). 
Oxygen*) was very suitable for this first investigation as it remains 
liquid down to a very low reduced temperature (0.30), and with 
the cryostat and the thermometrie apparatus that had been prepared 
for the investigation of the isotherms we have mentioned, we were 
able to trace the diameter from the critical point down to — 217° C. 
(reduced temp. 0.36). | 
If one were to leave neon and helium out of the question as 
affording too many difficulties for an investigation’ of this kind, then 
liquid densities at still lower temperatures could be obtained with 
hydrogen only. As yet, however, a cryostat for hydrogen is wanting 
1) KE. Marnras. Liege Mém. Soc. Roy des Sc. 2 (1899). 
2) The results were already given in the excerpt published in C.R. July-Aug. 1910. 
3) Measurements of the density of liquid oxygen were announced Comm. N°. 955 
(1906) pg. 28 note L, 
