260 
MR. J, LARMOR OX A DYNAMICAL THEORY OF 
electric mechanical forcive to be transmitted across the material medinm. The 
argument applies with suitable modification to any isotropic medium, as well as to a 
gas; for an seolotropic solid the specification of the actual molecular stress of 
different orioin which thus balances for each element the molecular electric forcive 
Avill be more complicated, involving the axes of molotropy as well as the axis of 
polarization 
47. For present purposes the important consequence is that, under circumstances 
of equilibrium, that part of the forcive on an element of a material body which arises 
from the excitation of neighbouring molecules and is expressed in terms of them 
alone, is not transmitted by material stress, but forms a balance on the spot with the 
cognate internal molecular forclves of other types."^ The only circumstance that 
might apparently vitiate this conclusion would be that the transitions between 
different media may be too abrupt to be treated, from the point of view of individual 
molecules, as really gradual transitions, after the manner of the above analysis; but 
even if we could imagine such a case, the discrepancy must for fluids be made up, 
provided the interface is a permanent one, by capillary forces in the interfacial layer, 
the effect of an outstanding surface derangement of energy. 
In a dielectric body situated in an electric field there is thus the mechanical strain 
due to the field; and there are also intrinsic change of volume and other dimensions 
and of physical properties, proportional to the square of the local polarization. If 
the dielectric is solid, those changes of dimensions may not fit in with the continuity 
of the material without the intervention of secondary strains ; but in fluid media the 
case is simple and precise, as no strain other than mere compression can exist. 
TJte Mutual Compensation of Local Molecular Agencies: Organized and Unorganized 
Energy: The Single Postidate of Thermodynamics, Avcdlcdde and Degraded 
Energy : Physical Basis of the Idea of Temperature. 
48. The scope of these molecular considerations (§§ 43-47) is wider than the special 
problem of polarization by which they are here precisely illustrated. To an intellig¬ 
ence that could follow the play of interaction between the individual molecules of 
* This principle of compensating molecular forcives was briefly enunciated for capillary action and 
applied by Young in his Memoir “ On the Cohesion of Fluids ” : ‘ Phil. Trans.,’ 1805. It forms the 
basis of Poisson’s “Nouvelle Theorie de I’Action Capillaire,” Paris, 1831, in which the attraction 
between the molecules of a fluid is balanced by a repulsion of much smaller range, supposed to be due 
to their caloric ; cf. especially Ch. VII. Cf. also Lord R.\yleigh “On the Theory of Surface Forces,” 
‘Phil. Mag.,’ 1883, 1890, 1892, especially 1892 (1) pp. 209-220: and Y.4.N der Waals’ “Essay on 
Continuity of the Liquid and Ga.seous States.” In these illustrative discussions, in which the 
intermolecular forces are restricted to a non-polar chai’acter, the compensating stress is usually found 
in the assumption of an intrinsic fluid pressure of range much shorter than that of the attractions 
between the molecules: the principle however in its general form only asserts that this compensation 
must exist, and there is no necessity to specify its character. 
