17G 



Prof. G. G. Stokes. On a remarkable [Feb. 20, 



Section I. — Preliminary Physical Examination. 



1. It will be necessary to premise that chlorate of potash belongs 

 to the oblique system of crystallisation. The fundamental form may 

 be taken as an oblique prism on a rhombic base, the plane bisecting 

 the obtuse dihedral angle of the prism being the plane of symmetry. 

 Hammelsberg denotes the sides of the prism by P, and the base by C, 

 and gives for the inclinations of the faces PP=104° 22' and CP= 

 105° 35'. The face C, which is perpendicular to the plane of sym- 

 metry, is so placed as to bring three obtuse plane angles together 

 at two opposite corners of the parallelopiped. The salt usually forms 

 flat rhombic or hexagonal plates parallel to the C plane, the edges of 

 the rhombus being parallel to the intersections of the P faces by the 

 C plane, and the hexagons being formed from the rhombic plates by 

 truncating the acute angles by faces parallel to the intersection of the 

 C plane by the plane of symmetry. 



The plane angles of the rhombic plates, calculated from the num- 

 bers given by Hammelsberg, are 100° 56' and 79° 4', while the 

 hexagonal plates present end-angles of 100° 56' and four side-angles 

 of 129° 32'. These angles are sufficiently different to allow in most 

 cases the principal plane of a plate, or even of a fragment of a plate, 

 to be determined at once by inspection. But in any case of doubt it 

 may readily be found without breaking the crystal by examining it in 

 polarised light. There are good cleavages parallel to the two P planes 

 and to the C plane. The crystals are very commonly twinned, the 

 twin plane being C. 



2. If one of the brilliantly coloured crystals be examined by reflec- 

 tion, and turned round in its own plane, without altering the angle of 

 incidence, the colour disappears twice in a complete revolution. The 

 vanishing positions are those in which the plane of incidence is the 

 plane of symmetry. The colour is perhaps most vivid in a perpendi- 

 cular plane ; but for a very considerable change of azimuth from the 

 perpendicular plane there is little variation in the intensity of the 

 colour. There is no perceptible change of tint, but on approaching 

 the plane of symmetry the colour gets more and more drowned in the 

 white light reflected from the surface. 



3. If instead of altering the azimuth of the plane of incidence a 

 plane be chosen which gives vivid colour, and the angle of incidence 

 be altered, the colour changes very materially. If we begin with a 

 small angle the colour begins to appear while the angle of incidence 

 is still quite moderate. What the initial colour is, varies from one 

 crystal to another. As we increase the angle of incidence the colour 

 becomes vivid, at the same time changing, and as we continue to 

 increase the angle the change of colour goes on. The change is 

 always in the order of increasing refrangibility ; for example, from 



