On Sulphate of lodo-quinine or Herapathite. 367 



The last two completely decided their complementary character, 

 for whilst the /3-prisms were " black," " red " or " blue," when 

 the planes of their length were vertical, the a-prisniswere " black," 

 " red " or " blue " whilst the planes of their length were hori- 

 zontally placed. PI. III. diagram II. figs. 1, 2, 3, a & /3. 



4. But by mixing the two prisms upon one slide their perfectly 

 complementary character was completely demonstrated, for when 

 an a-prism was crossed at right angles by a /3-prisru, the square 

 space where they overlapped was still transparent to common 

 light ; but if a /3-prism were overlapped by an a-prism, the length 

 of the two being parallel to each other, the oblong space where 

 they overlapped became black from their doubly absorbent pro- 

 perties ; in short, the optical examination fully proved the com- 

 plementary character of these crystals length for length. PL III. 

 diagrams II. IV. V. & VI. 



It was further shown that two varieties of hexagons existed; 

 those produced from strong spirituous solutions were longer than 

 broad, and possessed two acute angles of 65° and four equal 

 angles of 147° 30'. Examined by a vertically plane-polarized 

 beam, these were " black " when a line passing through the long 

 diameter or acute angles was parallel to the plane of primitive 

 polarization : these the author called " acute " or /3-hexagons. 

 The hexagons produced from acetic acid solutions, on the other 

 hand, were " obtuse " in their character, and had two angles of 

 115° and four equal angles of 122° 30'. These, when examined 

 by vertically plane-polarized light, were always " black " when 

 a line passing through the two angles of 115° was perpendicular 

 to the plane of primitive polarization : the author called these 

 "obtuse" or a-hexagons. PI. III. diagram I. a, /3. 



The rhombic crystals which presented themselves were also 

 " black " (examined in the same way) when their long diameters 

 lay parallel to the plane of primitive polarization, but transparent 

 when they were rotated 90°. PI. III. diagram I. figs. 1,2,3,&VI. 



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 maximum 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 careful measurement of the angles of the 

 various crystalline forms and by the results of their optical ex- 

 amination, it appeared that they may all be obtained from the 

 right rhombic prism, the acute angle of which is 65° and the obtuse 



