MR. MACQUORN RANKINE ON THERMO-DYNAMICS. 
117 
volume and pressure at which the actual heat is the same ; then are the points A and 
B situated on the same isothermal curve QQ. 
On the other hand, let the substance be 
allowed to expand from the volume and press- 
ure V A , P A , without receiving or emitting heat; 
and when it reaches a certain volume, V c , let 
the pressure be represented by P c , which is 
less than the pressure would have been had the 
actual heat been maintained constant, because, 
by expansion, heat is made to disappear. Then 
C will be a point on a certain curve NN pass- 
ing through A, which may be called a Curve of 
No Transmission. 
It is to be understood that, during the process last described, the potential energy 
developed during the expansion, and which is represented by the area ACV c V a , is 
entirely communicated to external substances ; for if any part of it were expended in 
agitating the particles of the expanding substance, a portion of heat would be repro- 
duced by friction. 
If o o o be a curve whose ordinates represent the pressures corresponding to various 
volumes when the substance is absoletely destitute of heat, then this curve, which 
may be called the Curve of Absolute Cold, is at once an isothermal curve and a curve 
of no transmission. 
So far as we yet know, the curve of absolute cold is, for all substances, an 
asymptote to all the other isothermal curves and curves of no transmission, which 
approach it and each other indefinitely as the volume of the substance increases 
without limit. 
Note. — The following remarks are intended to render more clear the precise 
meaning of the term Total Actual Heat. 
The Total Actual Heat of a given mass of a given substance at a given tempera- 
ture, is the quantity of Physical Energy present in the mass in the form of Heat 
under the given circumstances. 
If, for the purpose of illustrating this definition, we assume the hypothesis that 
heat consists in molecular revolutions of a particular kind, then the Total Actual 
Heat of a mass is measured by the mechanical power corresponding to the vis viva 
of those revolutions, and is represented by 
^2. mv 2 , 
m being the mass of any circulating molecule, and v 2 the mean-square of its velocity. 
But the meaning of the term Total Actual Heat may also be illustrated without 
the aid of any hypothesis. 
Fig. 2. 
