228 Proceedings of the Eoyal Society 
form, optical, electrical, and magnetic polarity, &c., the general 
aspect of this order of facts seems to me so marked and peculiar, 
and at the same time so unchemical, that I consider myself justi- 
fied in vindicating, for their scientific conception and interpretation, 
the existence of a principle sui generis ^ — a principle which, in 
virtue of its characteristic mode of action, I hold to be capable of 
modifying the chemical and physical energies of every part of a 
given atomic system, and all ^ this, be it well observed, with the 
entire preservation of the original arrangement. Let me, in illus- 
tration of my meaning, assume, in the first place, that the so-called 
atoms, — a definite number of which I conceive to he always and 
inseparably united according to some fixed principle of grouping, 
and thus to constitute an elementary atomic system or mole- 
eule, — are endowed with repulsive energies directly proportional to 
the range and intensity of their own vibratory movements, which 
latter may he supposed to consist in a series of periodical expan- 
sions and contractions of each spheroidal atom ; let me assume, in 
the second place, that under the influence of a certain physical 
agent the whole, or a given portion, of the atoms composing a 
molecule may experience a suspension or restoration of their vi- 
bratory movements ; and let me assume, in the last place, that the 
specific volumes of substances in the liquid and solid state depend 
upon and correspond to the degree of repulsive energy thus deve- 
loped in contiguous conflicting molecules ; then, by means of these 
three assumptions, we have established a nexus causalis between the 
variations in the specific volume of dense molecules and between 
the intensity of their repulsive energies. Nowit seems to me very 
clear that, so long as the peculiar force, which causes these varia- 
tions, and which I will provisionally distinguish by the icxm jparalytic 
force^ happens to induce precisely the same changes upon any two 
opposite sides of a given atomic system, the physical symmetry 
of that system will not be disturbed, but, on the contrary, that, 
when these changes differ, physical symmetry can no longer exist, 
and that the natural result will be the formation of an ethereal 
current from the stronger towards the weaker side during the act 
of atomic expansion, and of another current in the contrary direc- 
tion during the act of atomic contraction. Applying this view to 
the phenomena of circular polarisation, I have no hesitation in 
