( 681 ) 
to @ — we shall call this @' — to have a fixed position in space, 
Let @ be a lapse of time, consisting of a large number of periods, 
and consider, for this interval, the change of the amount of energy, 
contained within C. Let the cylinder be of so great a length A, that, 
if v should be negative, the disk cannot reach the plane @', before 
the end of the time @, and let A at the same time be so small in 
comparison with the distance 7, that terms which are of the order 
h ; Nd sagt tev 
— with respect to the quantities we are considering may be neglected. 
T . 
Then we need not trouble ourselves about the difference between the 
values of U for @ and @'; neither will it be necessary to attend to 
the flow of energy through the cylindrical surface of C. 
If, for the time @, ej is the amount of energy, by which the plane 
@' is traversed, e the increment of the energy, contained within the 
cylinder, and es the work done by the pressure exerted on w, the 
absorption is evidently given by 
e= EC — & — eg. 
Now: 
ej =cUad, 
and, the volume of the cylinder being increased by v@J@, 
eg=v Uob. 
Finally we have, since the displacement of the disk is v 0, 
eg —vUw. 
The result is therefore 
e—(ec—2v) Ua, 
or, by (3), if we continue to neglect terms in 2”, 
nt a? 
independent of the velocity of the earth. 
Physics. — “Ternary systems’. III. By Prof J. D. VAN DER WAALS. 
(Continued from page 560). 
14 Al 
The quantity (é)» occurring in equation (1) as a factor of re 
is negative for normal substances. It represents (Cont. II, pag. 101 
and following pages) the decrease of energy per molecule, when we 
have a finite quantity of the first phasis and an infinitely small 
quantity of the second phasis, and when we then make the substance 
fill the volume homogeneously, keeping volume and temperature 
constant; so we may also say that it represents the heat which in 
45* 
