638 REPORT — 1894'. 



ing tlieir admiration of the accuracy of hia observations and descriptions, more 

 especially when regard is had to the extreme simplicity of the apparatus available, 

 in those early days. 



It was recognised by Brewster that some of these optical anomalies are due to 

 a condition of strain of the crystal, as in the case of the diamond. But in other 

 minerals, as analcime and apophyllite, the hypothesis of strain was not entertained 

 bv him : he regarded the crystals as being truly composite and not simple ; and, 

 recognising optically different kinds of apophyllite, went so far as to give to one of 

 them the specific name of tesselite by reason of its distinctive characters. Biot, on 

 the other hand, sought to account for this kind of optical behaviour in another 

 ■way, by the hypothesis of lamellar polarisation : a crystal of alum, for example, he 

 held to be built up of thin laminae arranged parallel to the octahedral planes, and 

 imagined that light which has traversed such a crystal is polarised by its passage 

 through the aggregation of laminae in the same way as by pas.sage through a pile 

 of glass plates. But in the latter case there is a frequent passage of the light from 

 air to glass and glass to air, whereas in the case of alum there is no evidence of the 

 existence of atmospheric intervals. Frankenheim sought to overcome this diffi- 

 culty by the further hypothesis that the successive layers of a composite potassium- 

 and ammonium-alum are of different chemical composition, but such a difference of 

 material would be insufficient for the desired object by reason of the nearness to 

 each other of the refractive indices of alums of different composition. Still, it is a 

 remarkable fact that neither a pure potassium-alum nor a pure ammonium-alum 

 shows any depolarisation-eft'ects at all ; these belong only to the alums of mixed 

 composition, and yet there is no visible difference in the physical structure of the 

 crystals of simple and composite material. 



An epoch was made in the history of the so-called optical anomalies by tlie 

 publication in 1876 of an elaborate memoir by Pi'ofessor Ernest Mallard of Paris, 

 whose death last month deprived Mineralogy of its greatest philosopher. To make 

 the position more clear, we may take as a definite illustration the mineral boracite. 

 In development of faces and magnitude of angles the crystals of this mineral are, 

 as far as measurement with the goniometer can decide, precisely cubic in their 

 symmetry. But an apparently simple crystal of boracite, when examined in 

 polarised light, behaves exactly like a regularly composite body. If the crystal be 

 a rhombic dodecahedron in external development, all the twelve pyramids which 

 can be formed by drawing lines from the centre to the angular points are found to 

 be exactly similar to each other in everything but orientation ; and, further, each of 

 them has the optical characters of a biaxal crystal, the optic bisectrix of each 

 individual pyramid being perpendicular to the corresponding base, and thus having 

 a different direction for each of the si.x pairs of parallel faces of the dodecahedron. 

 Hence Mallard inferred that boracite belongs i-eally, not to the cubic, but to the 

 orthorhombic system, and that its crystallographic elements are so nearly those of 

 a cubic crystal that the molecular structure is m stable equilibrium, not only when 

 different molecules have their similar lines parallel, but also when only approxi- 

 mately similar lines have the same orientation ; further, the cubic symmetry of 

 the external form was regarded by him as a consequence of the approximation of 

 the crystallographic elements to those of a cubic crystal and of the variety of 

 orientation of the constituent molecules. Variety of orientation of constituent 

 molecules is, in fact, already recognised in the case of ordinary interpenetrant 

 twins. The variation of optical character in different crystals of the same 

 substance or different parts of the same crystal was then explained as being due 

 to the variation in the number of molecules belonging to each mode of orientation. 



According to another view, it was contended that a crystal of boracite is really 

 cubic and simple, but that, like unannealed glass, it is in a state of strain related 

 to the external form. It was replied that the optical characters of such 

 unamiealed glass are changed with the change of strain which follows the fracture 

 of the specimen, while those of boracite are unaltered when the crystal is broken. 

 To this it was rejoined that a once compressed gum retains its depolarising cha- 

 racter unchanged on fracture of the specimen, and that the same permanence may 

 very well be a character of some strained crystallised bodies. 



