( 175 j 
Physics. — “Isotherms of monatomic gases and their binary mixtures. 
III. Data concerning neon and helium. x> By Prof. H. Kamer- 
lingh Onnes. Communication N°. 112 from the Physical 
Laboratory at Leiden. 
§ 1. Critical pressure and triple-point pressure of neon. 
Mr. G\ Claude of Paris had with extreme kindness placed at my 
disposal a considerable quantity of a gas rich in neon which had 
been obtained by fractional distillation of air, carried out by the 
Paris Societe de l’Air-liquide according to Claude’s method. By means 
of my hydrogen circulation I was in a position to prepare from this 
a quantity of pure neon quite sufficient for the determination of 
various physical properties of this gas which, up to the present, has 
been but little investigated. When the pure neon was first collected 
in the mercury pump, a characteristic light phenomenon was at once 
apparent; this phenomenon seemed to have been unknown up till 
that time, although, as appeared from a paper published shortly 
afterwards, it had been observed by Prof. Collie, who also was 
indebted to the kindness of Mr. Claude, and had been further in¬ 
vestigated by him. The phenomenon is this: — whenever the mercury 
sinks from the reservoir of the TbPLER-pump to the lower tube, the 
meniscus is surrounded at the contact with the glass, by a narrow 
ring of clear orange light. By this neon can be immediately recognised. 
The triple-point pressure and the critical pressure were determined 
in the following manner. Pure neon was condensed in a glass tube 
which a steel capillary connected with a manometer tube and a 
valve through which gas from a reservoir could be admitted. The 
condensation was easily brought about by immersing in liquid hydrogen 
the experimental tube, surrounded by a protecting tube that served 
to lessen heat-conduction and thus to delay the temperature changes 
in the contents of the tube. Then by allowing the neon to flow 
quickly into the apparatus it was obtained in the liquid state in the 
experimental tube. If the protecting tube is kept surrounded by 
liquid hydrogen, then the experimental tube cools slowly, and after 
a time the neon begins to solidify. If, on the other hand, the liquid 
hydrogen surrounding the protecting tube is sucked away the tem¬ 
perature of the experimental tube slowly rises, and if, by means of 
the valve, the quantity of neon in the tube is well regulated, the 
meniscus is seen to disappear somewhere in the tube. By allowing 
the gas to expand through the valve the meniscus can be made to 
re-appear in the middle of a mist, just as it happens in ordinary 
determinations of the critical state. When the meniscus disappeared 
