MR. MACQUORN RANKINE ON THERMO-DYNAMICS. 
175 
which, when the difference between the higher and lower temperatures diminishes 
indefinitely, is reduced to the following : — 
K l +^=K„, (86.) 
that is to say : — 
Corollary. — Theorem. When a vapour is a Perfect Gas, and very bulky as com- 
pared with its liquid, the rate of increase of the total heat of evaporation with tempera- 
ture is nearly equal to the specific heat of the vapour at constant pressure. 
This was demonstrated by a different process, in a paper read to the Royal Society 
of Edinburgh in 1850. It has not yet been ascertained, however, whether any vapour 
at saturation approaches sufficiently near to the condition of perfect gas to render 
the theorem applicable. 
(53.) Concluding Remarks. 
In conclusion, it may be observed, that the Theory of the Expansive Action of Heat 
embodied in the propositions of this paper contains but one principle of hypothetical 
origin; viz. Proposition XII., according to which the actual heat present in a substance 
is simply proportional to its temperature, measured from a certain point of absolute 
cold, and multiplied by a specific constant; and that although existing experimental 
data may not be adequate to verify this principle precisely, they are still sufficient to 
prove, that it is near enough to the truth for all purposes connected with Thermo- 
dynamic Engines, and to afford a strong probability that it is an exact physical law. 
