370 Mr. William Phillips on the Otcyd of Tin. 



edges a b and c d, be represented by fig. 193, and the dotted line 

 e f will represent the line of junction of the upper and lower parts 

 1, 2, 1 and 1, 2, 1 of fig. 192, and consequently also the plane of 

 section abed, of fig. 190, which being parallel with the secondary 

 edge ef of that figure, is therefore parallel with the planes of the 

 primitive form, as a reference to figures 18 to 27, PI. 16, will 

 evince. In place of the edge a b, fig. 192, which is the edge of 

 the secondary pyramid, the primitive plane P is seen on fig. 218, 

 PI. 25, which plane gives on its opposed plane P of the same figure 

 (not visible on the figure, but which, as it w^ere, replaces the edge 

 c d Qii fig. 192) by the reflecting goniometer an exact incidence of 

 180°. It follows of course that the edges a b and c dy fig. 193, are 

 parallel with each other ; and also that the intermediate line of 

 section e f must be parallel with each, and therefore with the edge 

 of the secondary pyramid e fy fig. 190. 



For the discovery of the construction of that made of the oxyd 

 of tin, which, when viewed in the direction in which it generally 

 occurs, and in which it is delineated by fig. 188, PI. 24, appears to 

 take the form of a dodecahedron with triangular faces, I was prin- 

 cipally indebted to the direction of the strips on its planes. 

 Having noticed them to be mostly visible as described on that figure, 

 a suspicion arose that this made was composed of equal parts of 

 the prism formed by the planes of the first modification, and I 

 found by the common goniometer that the incidence of any plane 

 of the upper, on its connected plane on the lower pyramid, exactly 

 corresponded with that of 1 on 1, fig. 27, being 90". The idea of 

 its being composed of similar and equal portions of several crystals, 

 was further corroborated by observing, in almost every instance, 

 their natural joints along the edges from one apex to the other. 



This apparently dodecahedral made, fig. 188, PI. 24, at first 



