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Physics. — H. KaAMrRLINGH Onnes and W. Hruse. “On the measu- 
rement of very low temperatures. V. The expansion coefficient 
of Jena and Thüringer glass between + 16° and — 182° C.” 
Communieation N°. 85 from the Physical Laboratory at Leiden. 
(Communicated in the Meeting of June 27, 1903) 
§ 1. At Leiden the hydrogen thermometer (cf. Comm. N°. 27 
May ’96) is taken as the standard for very low temperatures. 
To reach the degree of accuracy otherwise obtainable with this, it is 
necessary to know the expansion coefficient of Jena glass 16™ to 
about 1°/,. Hence we have determined the two coefficients in the 
quadratic formula assumed for the linear expansion of glass below 
0°’ C. At the same time we have, in precisely the same circumstances 
made a similar determination for the Thüringer glass, from which 
the piezometers mentioned in Comm. N°. 50 (June 99), N°. 69 (April 01), 
and N°. 70 (May ’O1) were made, in order to be able to calculate 
and apply the correction for expansion to the results attained with 
these piezometers. 
Some time previously we made measurements on expansion coeffi- 
cients, among others on platinum. The value for this metal was 
required for the reduction, from the measurements mentioned in 
Comm. N°. 77 (Febr. ’02), of the galvanic resistance at low temperatures. 
But the results which we have lately obtained for the two above 
mentioned kinds of glass appear to us to be the first that are worth 
to be published; the final reduction of the measurements named 
above was postponed till the required accuracy was reached. However 
the measurements on platinnm must be repeated. 
Although the field of measurements at low temperatures is hardly 
touched, still we consider that in this field preliminary and approx- 
imate values are worth little. In the majority of cases approximate 
values of this kind can be obtained by extrapolation, and thus only 
those determinations which are accurate enough to allow a judgment 
on the question whether such an extrapolation is allowed or not, 
are really of use in advancing our knowledge. We have hence 
arranged our observations on the expansion coefficient so as to reach 
an accuracy of 200: 
For general the investigation of expansion at low temperatures it 
will be required to determine on the one hand the linear coefficient 
of solids and on the other the absolute coefficients for those substances, 
which remain liquid to very low temperatures, e. g. pentane, in such 
an hydrostatic manner as DuLone and Prrit’s (improved by RrGNAULT). 
The determination of the relative expansion of the liquid chosen can 
