868 Dr. Herapath's Researches into the Properties of 



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.7Twooi> -^it-' ^w the quantity for P" not being absolutely deter- 

 minable in consequence of its variability and minuteness. PI. III. 

 diagram VII. fig. 1. 



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 polarization, 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. PI. III. diagrams I. II. & VI. 



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 sjririt the ^-truncation ; and if 

 the two opposing forces of water and spirit were made equal in 

 intensity, they neutralized each other, so that the pure primary 

 rhombic prism resulted without truncation. 



It was further shown that hyponitric aether developed other 

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

 and 105° by truncation planes upon *he 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 thepolarized light as before. PI. III. diagram VII. fig. 2. 



As from the examination of certain rectangular quadrilateral 

 prisms of the a and ft 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 a , the author inferred that the development of 

 their optic axes could not be reasonably expected in these direc- 

 tions. He considered that the biaxial systems of rings would be 

 found to exist in thicker crystals ^when discovered), having 

 triangular replacement planes upon the solid angles, either of 

 the shorter or longer oblique diameters of the right rhombic 

 prism, and the results of these optic axes would be seen in a di- 

 rection perpendicular to these surfaces of replacement. PL III. 

 diagram VII. fi°rs. 3 & 4. 



