On Material Molecules and the Ether ial Medium. 253 
ditioiis differ under which the crystals are formed. In con- 
chision, there appears to he no established exception to the 
law, that the arrangement of the material molecules in space 
is definite for a substance of definite composition, and that, 
therefore, any change in chemical composition must be 
attended by a change in the arrangement of the molecules, 
and also in their mass, and hence by a change in optical 
properties. 
It may be objected that the variations in the angle be- 
tween the optic axes, observed by De Senarmont, are too 
large to be accounted for by the small changes in the ar- 
rangement of the molecules, which could be produced by 
isomorphous replacement ; but, a priori, there is nothing to 
show that very small changes in position of the molecules, 
especially if the forces exercised by the molecules on the 
etherial medium are sensible only at insensible distances, 
as without doubt they are, may not produce a change in 
the magnitude and direction of the resultant action on the 
elements of the ethereal medium, by no means insensible in 
its effect on the indefinitely small vibrations of the ether. 
That a minute change in the position of the molecules does 
produce a change in optical properties, is shown by the 
effect of pressure in producing double refraction in jelly, 
fluor-spar, &c. 
But if the optical properties of crystals are really caused 
by an action between the material molecules and the etherial 
medium, then the special optical properties of a given crystal 
should depend on the special arrangement of the molecules 
in that crystal, as shown by the form of the crystal, and 
expressed crystallographically by the system to which it 
belongs, and by the angles between its faces, that is, by 
the magnitude of its angular elements. In other words, 
some connection ought to exist between the form and optical 
properties of crystals ; and it ought to be possible to express 
the latter in terms of the angular elements of the crystal, 
and certain other constants dependent on the constitution 
of the ether, &c. That such a connection does exist, few 
physicists would, I think, be found to deny. De Senarmont, 
however, drew from his experiments on the variation of the 
