310 Mr. J. Rose-Innes on the Practical Attainment of 
has been determined by M. Chappuis (Trav. et Mem. du Bur. 
Int. xiii. pp. 21-25). 
The experiments of M. Amagat on the compressibility of 
nitrogen were conducted chietiy at high pressures ; but 
there is one measurement of pv at atmospheric pressure on 
the isothermal of 16° C, so that we may obtain the desired 
value of ( ~- ) at this temperature by means of an inter- 
polation. In the earlier part of this paper I gave —0*00035 
as the value founded on M. Amagat's experiments : and this 
figure is very nearly in accord with that subsequently 
found by Lord Rayleigh from his own experiments, viz., 
-0-00034 (Proc. Roy. Soc. lxxiii. no. 490, p. 153). The 
small difference between these two estimates is not sufficient 
to cause any appreciable change in the final numerical 
results, so that I have simply retained my former value of 
(i±\ 
\dp) t - 
M. Chappuis's most trustworthy determinations of the 
coefficient of expansion were made with a thermometer 
having a reservoir of iridio-platinum. His observations with 
this instrument may be divided into two groups : the first 
group having a mean pressure of 1*001855 m. of mercury 
gave a value of the coefficient 0*00367315, the second group 
having a mean pressure of 1*386787 m. of mercury gave a 
value of the coefficient 0*00367775. 
The value of the Joule-Thomson effect was determined by 
Joule & Kelvin for three initial temperatures, viz. : 7°*204 C, 
91°*415 C, and 91°*965 C. These last two temperatures are 
so close that we should not expect to find any marked 
difference in the Joule-Thomson effect as we pass from one 
to the other ; and. as a matter of fact, the cooling-effect per 
100 inches of mercury, for the actual gas employed, is much 
the same in both cases. But the figures which are given for 
the Joule-Thomson effect of pure nitrogen, as the result of 
an extrapolation, differ considerably in the two cases ; so 
much so that it is clear the difference cannot be due to the 
small change in the initial temperature, but that we are in 
presence of a serious experimental error. I have, as in the 
earlier part of this paper, altogether rejected the value at 
91°*965 C. on the ground that the experimental gas in this 
case contained a large percentage of oxygen, and that the 
figure given for pure nitrogen is the result of a considerable 
extrapolation. 
