of Edinburgh, Session 1871 - 72 . 
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latory motion: a portion of the energy of each corpuscule which 
has suffered collision must he supposed to be converted by the 
collision into vibrations, or vibrations and rotations. To simplify 
ideas, suppose for a moment the particles to he perfectly smooth 
elastic globules. Then collision could not generate any rotatory 
motion; hut if the cage-atoms constituting mundane matter be 
each of them, as we must suppose it to be, of enormously great 
mass in comparison with one of the ultramundane globules, and if 
the substance of the latter, though perfectly elastic, be much less 
rigid than that of the former, each globule that strikes one of the 
cage-bars must (Thomson & Tait’s “Natural Philosophy, § 301), 
come away with diminished velocity of translation, hut with the 
corresponding deficiency of energy altogether converted into vibra¬ 
tion of its own mass. Thus the condition required by Le Sage’s 
theory is fulfilled without violating modern thermo-dynamics ; and, 
according to Le Sage, we might be satisfied not to inquire what be¬ 
comes of those ultramundane corpuscules which have been in collision 
either with the cage-bars of mundane matter or with one another; 
for at present, and during ages to come, these would be merely an 
inconsiderable minority, the great majority being still fresh with 
original gravific energy unimpaired by collision. Without entering 
on the purely metaphysical question,—Is any such supposition satis¬ 
factory ? I wish to point out how gravific energy may be naturally 
restored to corpuscules in which it has been impaired by collision. 
Clausius has introduced into the kinetic theory of gases the 
very important consideration of vibrational and rotational energy. 
He has shown that a multitude of elastic corpuscules moving 
through void, and occasionally striking one another, must, on the 
average, have a constant proportion of their whole energy in the 
form of vibrations and rotations, the other part being purely trans¬ 
lational. Even for the simplest case,—that, namely, of smooth 
elastic globes,—no one has yet calculated by abstract dynamics 
the ultimate average ratio of the vibrational and rotational, to 
the translational energy. But Clausius has shown how to deduce 
it for the corpuscules of any particular gas from the experimental 
determination of the ratio of its specific heat pressure constant, to 
its specific heat volume constant.* He found that 
* Maxwell’s “ Elementary Treatise on Heat,” chap. xxii. Longman, 1871. 
