312 Scientific Proceedings, Royal Dublin Society. 
This interchange of atoms during the encounters is presumably 
the source, not only of such absorption spectra as that of chloro- 
chromic anhydride, but also of the bright spectra seen in Plicker’s 
tubes. 
If the gas be monatomic the spectrum is probably emitted only 
when the circumstances are such that two molecules can tempo- 
rarily coalesce into a diatomic molecule during the encounter, and 
become dissociated when the encounter is over; but in all cases 
where there is no ultimate change in the chemical constitution of 
the gas, its spectrum seems to be due to some event which is equi- 
valent to equal and opposite chemical reactions having taken place 
during either all or some of the encounters. 
An excellent way of helping us to appreciate the events with 
which we have to deal in molecular physics, is to conceive a model 
of them in which the durations shall all be enlarged 600 billions 
of times (6 x 10"). This particular magnification is found to 
have special convenience ‘attached to it.1. If prolonged to this 
extent, the most rapidly recurring motions in nature that are as 
yet known to us, viz.: those periodic events in a gas which give 
rise to the lines in its spectrum, would swing at rates comparable 
with the motions of the limbs of animals, and would have about 
as great a range from the swiftest of them to the slowest. On the 
same immense time-scale the duration of the journeys of the 
molecules of ordinary air would average about one day each, 
while the encounter which closes each journey may last some 20 
minutes.” The motions of the limbs of animals are able to accom- 
plish a good deal in a struggle lasting 20 minutes. On the same 
1 Wave-lengths of rays of light are usually expressed as fractions of a micron, and 
pendulums beating the same fractions of a second represent the corresponding etherial 
vibrations on the scale employed in the text. 
2 We may fill in this picture by combining a lengthening of distances with the 
prolongation of the times. A cubic millimetre, the volume of a small pin’s head, if 
each of its edges were magnified 10!° times, would become almost as huge as the earth. 
Under the same circumstances molecules of air would be spaced at intervals averaging 
ten metres; and 700 metres would have become the mean distance to which they 
would travel between their encounters. On this great scale, it would not be inappro- 
priate to use men or other animals to represent the individual molecules—their hearts 
beating, their chests heaving, their limbs in vigorous motion to represent the B or 
internal events; and as to the motions with which the molecules of a gas dart about 
amongst one another, these as they exist in common air would have become journeys 
as long and of as various lengths as the streets of a great city, while the widths of the 
