400 



to the plane of primitive polarization, but transparent when they 

 were rotated 90°. 



It was also shown that the doubly absorbent powers of the primary 

 rhombic crystal extended more or less on each side of the vertical 

 position, through an arc of 60°, so that upon rotating a crystal 

 through the whole circle, there were two arcs of 120°, in which the 

 crystals were more or less darkened, and two of 60°, each in which 

 the light passed through without loss; this transparency was of course 

 at its maxinum when the longer diameter of the rhombic plate lay in 

 a horizontal position, and gradually became less evident through an 

 arc of 30° above and below this line. By a carefal measurement 

 of the angles of the various crystalline forms and by the results 

 of their optical examination, it appeared that they may all be ob- 

 tained from the right rhombic prism, the acute angle of which is 

 65° and the obtuse 115°, the major axis being to the minor as 

 1'57 to 1, whilst the prism itself had scarcely appreciable length. 

 It was a mere rhombic scale ; but considered as a short prism, the 

 axes were 'P^-^f^^, Mp|-^, Tp^, the quantity for not being absolutely 

 determinable in consequence of its variability and minuteness. 



It appeared also that the a-prism and obtuse hexagon were the 

 results of truncation of the acute angles of the primary rhombic plate 

 or prism by planes at right angles to the plane of primitive polariza- 

 tion, when the crystals were " black," when examined by vertically 

 plane-polarized light ; whilst the /3-prism and acute hexagon resulted 

 from truncation of the obtuse angles of the same primary form by 

 planes parallel to that of primitive polarization, the crystals being 

 " black " when examined by a vertically plane-polarized beam ; and 

 that the octagonal and square plates, and rectangular parallelogram 

 resulted from the coincidence of these truncation planes in the same 

 individual form. 



It was shown that the solvent medium had the power of developing 

 these truncation planes, and it appeared that water produced the 

 a-truncation, and spirit the /3-truncation ; and if the two opposing 

 forces of water and spirit were made equal in intensity, they neu- 

 tralized each other, so that the pure primary rhombic prism resulted 

 without truncation. 



It was further shown that hyponitric ether developed other 

 crystalline forms, converting the rhombic plate into one of 75° 

 and 105° by truncation planes upon the acute angles of the primary 

 rhombic plate, cutting off portions equal to half the long axis, 

 and leaving the shorter or transverse axis untouched. By this 

 means the new rhombic crystal appeared " black " when the longer 

 diameter was at right angles to the plane of vertically polarized 

 light, as if it had been rotated through 90°, whilst it absorbed the 

 polarized light as before. 



As from the examination of certain rectangular quadrilateral prisms 

 of the a and (d varieties, it appeared that Herapathite possesses 

 doubly absorbent powers of nearly, if not perfectly equal intensity, 

 in directions coincident with all three rectangular axes P", M*" 

 and T", the author inferred that the development of their optic 



