236 
MR. J. LARMOR ON A DYNAMICAL THEORY OF 
jDolarizatioLi or* a molecule is upset by an eucountsr with another molecule, it is 
instantly restored to its normal value, as soon as the violence of the encounter is 
over ; so that, the relative times spent by the molecule in encounters being small in 
every case, they hardly affect the inductive capacity of the medium ; or in other 
terms, the density by itself liardly affects the molecular refraction equivalent (except 
in so far as the restoration of the steady state may involve absorption, § 28 infra\ 
and the constancy of the coefficient h is further justified.* 
22. T1 le molecular theory leads to the conclusion that the electric seolotropy of 
crystals in which the dielectric constant differs much from unity, may be in part due 
to the distribution of the molecules in space and in part to the orientation of the 
individual molecules ; and that therefore the same applies to double refraction. The 
intrinsic polarity which is revealed by pyroelectricity and piezoelectricity also shows 
that orientation is a real cause. But magnetic aeolotropy must practically be wholly 
due to orientation of the molecules, as the smallness of the susce 2 :)tibility makes the 
effect of arrangement inappreciable. The double refraction induced in dielectrics in a 
strong electric field is possibly mainly due to molecular orientation, as also that 
arising from meclianical strain. 
The difference of absorption in different directions in a crystal like tourmaline must 
be of an order of numerical magnitude not higher than the difference of the refractions: 
an easy computation shows that it is really of a considerably lower order. This crystal¬ 
line absorption can only be due to molecular orientation : it is of course excessively 
smaller than the absorption in metals, which is comparable with the whole refractive 
index ; it would not therefore sensibly affect the laws of reflexion. 
General Theory of Optical Dispersion. 
23. A formula for optical dispersion was obtained in § 11 of the second part of 
this menroir, on the simple hypothesis that the electric polarization of the molecules 
vibrated as a whole in unison with the electric field of the radiation. The kinetic 
molecule of § 11 supra, with its steady momenta, will however usually have various 
free periods, and as many absorption bands ; to take account of them, and also for 
other reasons which will appear, it is desirable to have a more complete dynamical 
theory. 
The problem of dispersion, in its general form, is thus that of the transmission 
* The analysis of this section does not agree with a theoretical investigation of the iudnctive capacities 
of mixtures of non-conducting liquids which do not exhibit change of volume in mixing, given by 
SiLBEKSTEiN (‘ Wicd. Aim.,’ 1895) ; his result is that K, or what comes to the same under these conditions, 
K — l,is an additive physical constant, Avhercas the formula of Clausius and Lokextz makes (K — 1)/(K-1- 2) 
aflditive. The determinations made by' SiLBEiiSTEiN for niixtuies of benzol and phenylethydacetate give 
lesults for the Lokentz constant which arc always in excess of the theoretical value, by- amounts ranging 
u]) to 8 per cent.; the discrepancies for his own con.stant K —1 arc rather smallci', and are in both diree- 
t ions. 
