1893-94.] Prof. Tait on Compression of Ordinary Liquids. 285 
On the Application of Van der Waals’ Equation to the 
Compression of Ordinary Liquids. By Prof. Tait. 
(Read June 4, 1894.) 
In a paper, read for me to the Society in January last {ante, p. 245), 
I pointed out the difficulties I had met with in trying to reconcile 
Van der Waals’ equation with Amagat’s experimental data for 
common liquids, and I promised to recur to the question when the 
state of my health should permit. I now find that, as I had then 
only surmised, the constants in Van der Waals’ equation necessarily 
become non-real when we try to adjust it to Amagat’s data. 
The proof of this assertion is very simple. Suppose the equation 
BT 
to hold for any three pairs of values of p and v ; say p and a , q and 
h , r and c . Eliminating BT among the three resulting equations, 
we have 
+ + ^ (g5+^-)-(rc + -) 
The values of A are therefore to be found from the quadratic 
^ _ %{pq{a -h)) = 0. 
Write, for brevity, 
-D ^ r\ ^ 
B = 
a - h 
so that one at least of P, Q, E is essentially negative, if p, q, r be 
all positive. The condition that the values of A shall be real is 
{ :S(P(a& -hc-\-ca))Y + 42(PQ(a - &) V) > 0 . 
