306 Scientific Proceedings, Royal Dublin Society. 
energy to the gas, and which thereby become depositories of 
energy. ‘These motions or events may be first divided into the 
two classes A and B, external and internal events. The A or 
external events are simply the motions of the centres of inertia of 
the molecules between their encounters: the B events are rotations 
or other motions, or changes of configuration, of the parts of a 
molecule relatively to one another, or electrical or other events : 
any events in fact which can be brought about by an expenditure 
of energy. These may all be spoken of either as motions or 
events, using the term motions in its generalised sense. 
Again, the B events require to be subdivided into three classes : 
Ba events, which readily exchange energy with the A events, 
7.e. which are affected by the speed with which two molecules 
plunge into one another when an encounter takes place, and 
which in turn contribute in a marked degree towards determining 
with what velocities they shall separate when the encounter is over. 
Tn contrast to these, the Be events (if any such exist, as is perhaps 
probable if the vortex theory of matter is true) are such as are 
completely isolated from the A and Ba events, and therefore 
neither gain nor lose energy in the encounters. Between the 
Ba and Bc events stand Bb events, which seem to be a con- 
spicuous part of what is actually going on in all the real gases 
of nature. These Bd events are not wholly unaffected by the 
encounters, but in any one encounter gain or lose but little 
energy; while after millions of encounters, the transference of 
energy, perhaps chiefly in one direction—from them to other 
events—may be appreciable. Events of this kind may produce 
even conspicuous effects in times which appear to us very short, 
since in the ordinary air about us each molecule meets with a 
million of encounters in something like the seventh part of the 
thousandth of one second of time. In attempting to interpret 
the results furnished by our dynamical investigations, the extra- 
ordinary‘ rapidity with which the molecular events succeed one 
another in the actual gases of nature must be fully allowed 
for. 
Another matter to be kept carefully in mind is that most of 
the dynamical investigations go on the assumption that interac- 
1 See the description of an illustrative model in the last paragraph of this paper. 
