MR. MACQUORN RANKINE ON THERMO-DYNAMICS. 
147 
of reasoning may be certainly true, while those of the former can never be more 
than probable ; for how complete soever the analogy between the laws of two classes 
of phenomena may be, there will always remain a possibility of the phenomena 
themselves being unlike. A hypothesis, therefore, is incapable of absolute proof ; 
but the agreement of its results with those of experiment may give it a high degree 
of probability. 
The laws of the transmission of radiant heat are analogous to those of the propaga- 
tion of a transverse oscillatory movement. The laws of thermometric heat are analo- 
gous to those of motion, inasmuch as both are convertible into mechanical effect ; 
and motion, especially that of eddies in liquids and gases, is directly convertible into 
heat by friction. If, guided by these analogies, we assume as a probable hypothesis 
that heat consists in some kind of molecular motion, we must suppose that thermo- 
metric heat is such a molecular motion as will cause bodies to tend to expand ; that 
is to say, a motion productive of centrifugal force. Thus we are led to the hypo- 
thesis of Molecular Vortices. 
This hypothesis, besides the principles already enunciated, of the mutual trans- 
formation of heat and motive power in homogeneous substances, leads to the follow- 
ing special conclusion respecting the 
Relation between Temperature and Actual Heat: — 
When the temperature of a substance, as measured by a perfect-gas thermometer, 
rises by equal increments , the actual heat present in the substance rises also by equal 
increments : — 
a principle expressed symbolically by the equation 
Q = fc(r— *), (35.) 
where Q is the actual heat in unity of weight of a substance, r its temperature, 
measured from the absolute zero of gaseous tension, x the temperature of absolute 
cold, measured from the same point, and & the real specific heat of the substance, 
expressed in terms of motive power*. 
The enunciation of this law was originally an anticipation of the results of ex- 
periment ; for when it appeared, no experimental data existed by which its soundness 
could be tested. 
Since then, however, one confirmation of this law has been afforded by the expe- 
riments of M. Regnault, showing that the specific heat of atmospheric air is sensibly 
constant at all temperatures and at all densities throughout a very great range; and 
another, by the experiments of Messrs. Joule and Thomson referred to in Proposi- 
tion VI., on the thermic phenomena of gases rushing through small apertures, 
which not only verify the theoretical principle, but afford the means of computing 
approximately the position x of the point of absolute cold on the thermometric 
scale. 
* The hypothesis of Mayer amounts to supposing that »,=0, or that the zero of gaseous tension coincides 
with the point of absolute cold. 
u 2 
