AA G. Jounstonse Stoney On Polarization Stress in Gases. 
it is, for a vast number of molecules, and for much jostling amongst them. 
The temperatures with which we are dealing are such that the average velocity 
with which the molecules of air dash about may be taken as 500 metres per second ; 
and the molecules meet with so many encounters that the direction of the path of each 
is changed somewhere about a unit-ten of times (10,000,000,000) every second. To 
complete the picture we must remember that each molecule is ina state of vigorous 
internal motion as well as travelling about among its fellows, and that, when an 
encounter takes place, the energy which passes from one molecule to another is 
employed in changing both these kinds of motion, and possibly (but not probably) 
another part becomes potential energy, i.e., energy expended im altering the 
configuration of the parts of the molecule, or the position of its parts with 
reference to the ether. The motions which go on within the molecules are 
what give rise to the linear spectra of gases, and are, therefore, those motions 
of the gas that act on the ether and are in turn partly controlled by it.* They 
are recurring motions which, at least in some cases, are resolvable either into har- 
monics like the vibrations of a string, or else into quasi-harmonics, not to be 
distinguished from harmonics by observation (See Donkin’s “Acoustics,” §194), 
like the transverse vibrations of an elastic rod—probably the former. On the more 
probable supposition that they are true harmonics, the periodic times have been 
determined with great precision in some cases, notably in the cases. of a motion 
within the molecules of hydrogen, which gives rise to three of its spectral 
lines, and a motion within the molecules of Chlorochromic Anhydride, which 
gives rise to 105 of its spectral lines. In hydrogen the motion is repeated as 
often as 2,280,000,000,000 times each second in every molecule, and in the vapour 
of Chlorochromic Anhydride, rather more than 800,000,000,000 times.t Such 
are the periodic times on the supposition that the motions are resolvable into true 
harmonics ; and whether the fact be that the components of the motions are 
harmonics or quasi-harmonics, their periodic times are at all events quantities 
of this order. The general presumption, therefore, is that the periodic times within 
* May we not look, with some prospect of success, to the control which is exercised by the ether on 
the internal motions of the molecule, for the explanation of the number of “ degrees of freedom” of a 
molecule, which (on the supposition that there is no potential energy) is in most gases 5 (see Watson’s 
“Kinetic Theory of Gases,” p. 39). The number 5 appears to indicate that the motions within the 
molecules are trammelled, as here suggested. This view is, moreover, supported by the fact that light is 
emitted by the gas, which could not be the case unless vast numbers of molecules moved in unison with 
one another, and the most probable account of this appears to be that they are all trammelled in the same 
way by their common relation to the ether. 
t The periodic times deduced from the observations are respectively ane and 5H6 r being the 
cos ? 
time that light takes to advance one millimetre in vacuo. (See Phil. Mag., April, 1871, p. 295, and July, 
1871, p. 45. In the former paper read 0:013127714 for 013127714.) The first of these determinations 
was made by the present author, and the second by the present author, in conjunction with Professor 
Emerson Reynolds, of Dublin; but before either of these determinations were made Professor Clifton, 
of Oxford, had mentioned at the Exeter meeting of the British Association in 1869, that he had found 
two of the hydrogen lines (probably C and h) to be related harmonically. I am not aware that any 
record of this important observation has been published. 
