252 Proceedings of the Royal Physical Society. 



the angular elements differ from 80' to 2°, so that when 

 these substances replace one another isornorphously, 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 maybe 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- 



