ox THE MEASUREMENT OF TEMPERATURE. 
107 
Correction of the Constant-Pressure Thermometer. 
Temperature of ice-point 
Gas. 
(reciprocal of 
volume coefficient). 
Correction. 
Absolute ice-point. 
Hydrogen. 
273-13 
-0-13 
273-00 
Nitrogen. 
273-41: 
0-72 
273-16 
Of the data employed in making the foregoing- calculation only the pressure 
coefficients of hydrogen and nitrogen, which have been determined by P, Chappuis, 
of the “Bureau International des Poids et Mesures,” and the variation of “p.v.” with 
pressure at the normal temperature, which has been recently investigated by Onnes 
(‘Leiden Laboratory Reports,’ 1901) in the case of hydrogen, can he considered 
as being known with sufficient certainty. Amagat has determined the variations of 
“ p.v.” for nitrogen, hut the experiment was made at high pressure and not at the 
pressure under which thermometric measurements are usually made. In view of 
the fact that both air and oxygen appear to obey Boyle’s law at pressures 
below 150 millims. of mercury (Rayleigh, ‘Phil. Trans.,’ A, vol. 198, p. 417), extra¬ 
polation of Amagat’s results at high pressure does not seem justifiable; and to 
obtain the data necessary for the correction of the gas thermometer the variations 
of “ p.v.” should he carefully investigated at pressures below two atmospheres. 
Regxault’s values for the volume coefficients of nitrogen and hydrogen are not 
to he relied ujion. 
The Joule-Thomson effect for air and hydrogen was detemiined by Joule and 
Lord Kelvin more than half a century ago. Without discussing their results, it 
may lie pointed out that by employing electrical methods for tlie measurement of 
temperature, it should now he possible to measure the effect with a much higher 
degree of accuracy, and over a wider range of temperature than was then possible 
An investigation in this direction has lieen commenced by Dr. R. A. Lehfeldt 
conjointly with one of us. 
2. Suitalne Thermometric Substances. 
For the reason that the composition of atmospheric air varies with the prevailing 
condition, air is not a suitable substance for use in thermometry. Atmospheric 
nitrogen appears, however, to contain a constant cpiantity of argon and its companions, 
and may be employed in thermometry over a range, limited, perhaps, by the tem¬ 
peratures at which it condenses on the one hand and dissociates on the other. 
Until recently, hydrogen appeared to be the most important thermometric substance. 
r 2 
