667 
Physics. — “On the rectilinear diameter for argon. By E. Marnras, 
H. KAMERLINGH Onnes and C. A. CROMMELIN. Comm. N°. 181a 
from the phys. Lab. at Leiden. 
§ 1. Zntroduction. The present paper forms a continuation of 
the investigation of the diameter for substances of low critical tempe- 
rature with which a beginning was made with oxygen.) The 
importance of this and of similar investigations was indicated in the 
introduction to the Communication referred to, so that we need not 
discuss the point further here. 
We chose argon for the present investigation since the isotherms 
for that gas had already been determined to within the neighbourhood 
of the critical point, while the critical pont itself, the vapour pres- 
sures and even preliminary values of the densities of the coexisting 
vapour and liquid phases were aiready known *) the monatomic 
nature of the gas, moreover, will undoubtedly enhance the value of 
the results. 
§ 2. Apparatus. The apparatus was essentially the same as that 
employed in the investigation of oxygen. The arrangement for com- 
pressing the argon and also the volumenometer have, however, 
undergone some modification since that time, so that it seemed 
desirable to take this opportunity of publishing a new diagram of 
the whole apparatus (Fig. 1). ; 
The modiications of the volumenometer and of the auxiliary 
apparatus belonging to it have already been described in full detail *). 
The use of such a costly gas as pure argon necessitated, however, 
a compietely new arrangement of the pressure connections. The 
copper tubes of which all the connections were made were chosen 
as narrow as possible so as to reduce the quantity of gas in the 
dead space to a minimum. The argon was contained in the steel 
cylinder A which was completely immersed in oil; so too were 
all the taps and coupling pieces which contained compressed gas. * 
Through the taps C,, and C,,, the gas passes to the spiral Sp; 
1) Proc. Febr. 1911. Comm. No. 117. C. R. 151, 213 and 477, 1910. 
2) Proc. May 1910, Comm. No. 115, Proc. Dec. 1910, Gomm. No. 118, C. A. 
CROMMELIN, Thesis for the doctorate, Leiden. 1910. 
8) Proc. May 1911 Comm. No. 121a, Proc. Sept. 1912 Comm. No. 127c and 
W. J. De Haas, Thesis for the Doctorate, Leiden 1912, in which diagrams of 
the modified volumenometer are also given. Certain small errors in these diagrams 
make it desirable to publish a diagram here in which these errors are corrected. 
4) Proc. June 1905 Comm. No. 94). The value of this device for the detection — 
of leaks has already been repeatedly emphasized 
