13 
1909-10.] Andrews’ Measurements of Compression. 
In a letter dated January 27, 1892, Professor Tait writes as follows 
to Miss Andrews : — 
“ In the course of my work on the compression of gases I have at last 
come to gaseous mixtures. Now, with the exception of air, which has 
been fully dealt with by Amagat, I know of no mixtures experimented on 
except those of which your father treated in his posthumous paper (p. 457 
of the memorial volume). The data are given in full there for a mixture 
of 3 vols. C0 2 and 4 vols. N. But (see p. 467, at foot) it is clear that 
another mixture in more valuable proportions, viz., 343 vols. C0 2 and 
1 vol. N, was also examined. The details of this, if they have been pre- 
served, would be very valuable ; indeed, I wonder that neither Stokes 
nor myself thought of asking about them at the time. ...” 
In the posthumous paper there is also another mixture referred to, 
viz., 6’2 vols. C0 2 and 1 vol. N. 
The data for both of these sets of experiment are now given in detail. 
It will be seen that the manometer gas was not hydrogen but air. The 
volumes in the first table are not given in cubic centimetres; but as a 
sufficient datum for the calculation of the pressures, the original volume 
at ordinary temperature and pressure is given in terms of the length of the 
containing tube. 
The following notes, prepared by Miss M. K. Andrews, will make these 
tables more easily intelligible to the reader. They should also be read in 
connection with Dr Andrews’ own account as given in the posthumous 
paper of 1886 (see Scientific Papers, pp. 457-471). 
With the mixture of 6*2 vols. C0 2 and 1 vol. N, Dr Andrews found that 
at the lower temperatures nitrogen is absorbed in the ordinary way, and the 
curvature of the liquid surface is preserved, so long as any portion of the 
gas is visible (Exp. 551-555) ; but at higher temperatures the liquid surface 
loses its curvature and is effaced by pressure alone (Exp. 575 et seq.). 
With the mixture of 3*43 vols. C0 2 and 1 vol. N, the critical tempera- 
ture was found to be 14° C., and the corresponding pressure about 98 atmo- 
spheres. This point was obtained by gradually lowering the temperature 
till it was just possible to obtain a small trace of liquid by the application 
of pressure. Experiments were made at lower temperatures than the 
critical point (14°) to fix the pressures at which for the same temperature 
the liquid first appeared and was afterwards effaced (Exp. 568-570, 571- 
573, 932-934). 
Dr Andrews was long perplexed with anomalous results, the carbon 
dioxide sometimes liquefying by the application of pressure above 20°, at 
( Continued on page 22.) 
