the Thermodynamic Scale of Temperature. 309 
of differential pressure, to 0°*100 C. at temperatures of 4° C. 
or 5° (\. and to o, 155 C. at temperatures of from 89° G. to 
93° C. The change of a. with the pressure was examined by 
M. Amagat (see Annates de Chimie et de Physique, 6 e serie, 
t. xxix. p. 127) ; it is unfortunate that his experiments apply 
to high pressures only, but since the value of a appears to 
vary steadily with the pressure over a considerable range of 
pressure, we may probably make use of his results without 
introducing any sensible error. The decrease in a. corre- 
sponding to an increase of 100 atmospheres pressure is 
•00018. The value of (-r-) can also be obtained from 
\dp ft 
M. Amagat's experiments (see his paper, Ann. de Chimie 
et de Physique, 5 e serie, vol. xxii.). In the first part of the 
present paper I practically accepted M. Amagat's statement 
that the isothermals of hydrogen, — yjr plotted against, p — 
form a set of parallel straight lines with a slope correspond- 
ing to (t^-) = '00078. Though M. Amagat warns his 
readers that this law is only approximate, I came to the 
conclusion that it was sufficiently accurate for the purpose I 
had in view ; accordingly I took (7^-) to be '00078, and 
attached this value to the temperature 50° (J. as being mid- 
way between the boiling- and freezing-points. But a more 
careful examination of M. Amagat's results has shown me 
that the rough method 1 employed was somewhat rougher 
than I supposed. The curvature of the isothermals — though 
not very apparent to the eve in a diagram — indicates a 
smaller deviation from Boyle's law for those portions of the 
isothermal wm'ch are nearer the axis of zero pressure; and 
I rind that the isothermal lor 17°" 7 C. yields a value of 
W it 
'000015 at low pressures. This value agrees very 
nearly with that given by MM. Leduc & Sacerdote, viz. *00064 
at 16° C. (see Comptes Bmdus, t. cxxv. 1897, p. 299), and is 
not Jar removed from the value given by Prof. K. Onnes, 
viz. '000623 at 20° 0. (Schalwijk, Academie des /Sciences 
d' Amsterdam, June 1901). 
Nitrogen. — The isothermal compressibility of this gas has 
been studied by M. Amagat (Comptes Rendus, t. xeix. p. 1153) ; 
the ga^ was subjected to the porous-plug experiment bv 
Joule & Kelvin (Kelvin's Reprinted Papers, vol. i. p. 421 ) ; 
and the coefficient of expansion at various constant pressures 
