1892 - 93 .] Prof. Tait on the Compressibility of Liqiiids. 
63 
On the Compressibility of Liquids in connection with 
their Molecular Pressure. By Professor Tait. 
(Read March 6, 1893.) 
That liquids, if finitely compressible, must (at any one tempera- 
ture) become steadily less compressible as the pressure is raised, 
seems to be obvious without any attempt at proof. Yet the 
assertion is even now generally made, mainly in consequence of an 
erroneous statement of (drsted, which has been supported by some 
comparatively recent investigations of Cailletet and others, that the 
compressibility of water (at any one temperature) is practically the 
same at all pressures not exceeding a few hundred atmospheres. 
But in 1826 {Fhil. Trans. ^ cvi.), Perkins had clearly established 
the fact that the compressibility of water at 10° C. diminishes : — 
rapidly at first, afterwards more and more slowly : — as the pressure 
is gradually raised. Perkins’ estimate of his pressure-unit seems to 
have been considerably too small, so that his numerical data are 
not very trustworthy : — but this does not in the least invalidate the 
proof he gives of the gradual diminution of compressibility; for 
that depends of course upon relative, not upon absolute, values. 
In the very earliest determinations which I made, some ten or 
twelve years ago, while examining the pressure-errors of the 
Challenger thermometers, this diminution of the compressibility of 
water was prominently shown: — and in 1888 I gave, as a fairly 
close approximation to the average compressibility for the first p 
atmospheres, the empirical expression 
in which the constants depend on temperature only. 
This, it will be observed, is in complete agreement with the form 
of the result of Perkins. I also found that the addition of common 
salt, to the water operated on, had the effect of increasing the 
constant B in this formula by a quantity proportional to the amount 
of salt added ; A being practically unchanged, so long as the tem- 
perature was kept constant. 
