252 Froceedinys of tlce Royal Fltysical Society. 
the angular elements differ from 30' to 2°, so that when 
these substances replace one another isomorphously, it is 
quite reasonable to suppose that changes, although no doubt 
slight ones, are produced in the crystalline form. This may 
be illustrated by the mineral alstonite, which may be looked 
upon as aragonite, in which half the carbonate of calcium is 
replaced by carbonate of barium, the angular elements of 
aragonite being thereby altered from 58° 5' and 40° 50' to 
59° 25'-5 and 38° 39', differing respectively by 1° 20'-5 and 
2° 11' from those of aragonite. We may say that the angular 
elements of aragonite have been altered to this amount by 
isomorphous replacement. 
It will be seen that the view we take is, that chemical 
composition has no direct effect upon optical properties, but 
that it influences them only inasmuch as the form of a 
crystal is determined by its chemical composition, and the 
optical properties are held to be directly due to the form of 
the crystal, or to the arrangement of the molecules. It may 
be argued, that the form of a crystal is not entirely deter- 
mined by its molecular constitution, as shown by the facts 
of dimorphism. We hold, however, that all the so-called 
cases of dimorphism can be explained by real, though small, 
differences in chemical constitution, and differences in tem- 
perature and pressure at which the crystals were formed, 
and that a substance of given chemical composition, when 
crystallised under the same physical conditions as to 
pressure and temperature, will always assume the same 
form. 
Take the case of titanic acid, which is said to be trimor- 
phous in anatase, rutile, and brookite. In these minerals 
the titanic acid is always associated with variable quantities 
of sesquioxide of iron. Or consider again the dimorphism 
of carbonate of calcium, as calcite and aragonite. Aragonite 
is never pure carbonate of calcium, but is always associated 
with variable quantities of the carbonates of lead, strontium, 
and manganese. Calcite contains substances like carbonate 
of iron, oxide of zinc, &c. In other cases of dimorphism, 
as of sulphur, and the artificial production of calcite and 
aragonite from pure carbonate of calcium, the physical con- 
