: ( 767 ) 
a nut carrying a glass cock unto 4;, evacuating and reading as before. 
In the same way .the area of the graduated tube h, was determined 
but with more precautions as to constancy of temperature, and of 
the large volume V; with the area of bz. The precautions as to 
constancy of temperature and accuracy of reading and weighing were 
always referred to a higher accuracy than the one actually sought. 
Physics. — Communication n°, 78> from the Physical Laboratory 
at Leiden: ‘“Jsotherms of diatomic gases and their binary 
mixtures. III. The isotherms of Oxygen at 20.°0 C.15.°6 C. 
0°.0 C.” (By Prof. H. KAMERLINGH Onnes and H. H. 
Francis HYNDMAN). 
(Communicated to the meeting of March 29, 1902). 
§ 10. The purpose of the research at ordinary temperatures. 
Measurements were made in this region for two purposes. 
10, In order to calculate our observation at low temperatures by 
equation (7) it is necessary to know the values of pv4 at the given 
pressures to at least the same accuracy as that sought for in the 
measurements at low temperatures. In Comm. n°. 69, § 6 we have 
given aaa as the accuracy aimed at and to this accuracy there 
are no data at present available. The most trustworthy measurements 
of AMAGAT!) commence at 100 At. and even should they have no 
constant error they do not admit of interpolation for the values of 
pva to this accuracy. 
2°, We wished to so test our apparatus at ordinary temperatures 
in a region where the results could be compared with and linked 
into the above mentioned measurements of AMAGAT, that we could 
satisfy ourselves of the accuracy of the method at low temperatures. 
Measurements were made at 15°.6 ©. to compare directly with 
AMAGAT, but the majority at 20° C. because it is a more generally 
useful temperature and can always be obtained. At the same time 
as the measurements at ordinary temperature we give a series at 
0.0 C. as this can also be directly compared with AMAGAT. 
With the help of these measurements we can improve the coeffi- 
cients in the series for pra given in Comm. n® 71 and can also 
obtain the expansion coefficients required for the small temperature 
differences t; and 74, more accurately than heretofore. 
1) Aun. de Chim, et de Phys, 1893, 
