MR. MACQUORN RANKINE ON THERMO-DYNAMICS. 
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represent the heat received by the substance during the operation ACB, and 
^V B 
I PdV=area V a ACBV b 
the power or potential energy, given out. 
Then the theorem of this article is expressed as follows : — 
Ha, b — jy Pf/V=Q B Q a +S b — S A = AQ- f A.S .... ( 2 .) 
being a form of the General Equation of the Expansive Action of Heat, in which the 
Potential of Molecular Action, S, remains to be determined. 
(6.) Second Corollary (see fig. 4). — The Latent Heat of Expansion of a substance, 
from one given volume V A to another V B , for a given amount of actual heat Q ; that 
is to say, the heat which must be absorbed by the substance in expanding from the 
volume V A to the volume V B , in order that the actual heat Q may be maintained 
constant, is represented geometrically as follows. Let QQ be the isothermal curve 
of the given actual heat Q on the diagram of energy ; A, B two points on this curve, 
whose co-ordinates represent the two given volumes and the corresponding pressures. 
Through A and B draw the two curves of no transmission AM, BN, produced indefi- 
nitely in the direction of X. Then the area contained between the portion of iso- 
thermal curve AB, and the indefinitely-produced curves AM, BN, represents the 
mechanical equivalent of the latent heat sought, whose symbolical expression is 
formed from Equation 2 by making Q B — Q A =0, and is as follows : — 
pV B 
H A) b (for Q= const.) = PdV+S B -S A (3.) 
MDCCCLIV. 
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