Mr, William Phillips on the Oxyd of Tin, 371 



sight, seems to have no analogy with the preceding macles, but 

 that it results from the same law of section as those described by 

 figs 1S6 and 187, may be readily shewn. Let the section a b c d^ 

 fig. 190, which is parallel with the edges ef and^ h of that figure 

 be represented by a section abed, fig. 194, parallel with the 

 edges e f and g h of that figure ; then let e b b d he a. section in 

 the opposite direction paiallel with the edges y^ and eg. By 

 placing the prism so that the edge /i b i of fig 194 shall be repre- 

 sented by /i b iy fig. 195, it will be seen that the lines of section 

 abed and cbhd are the same on each figure, and that by these 

 sections two equal portions bhda and bede are obtained from 

 the prism, the former of which is shewn by fig. 196 ; and it will 

 also be seen that the planes 1,1, of the latter figure, correspond 

 with those of 1, 1, fig 188. It will be understood therefore that 

 this made consists of a number of equal portions of the prism, 

 described by fig. 196, and that the planes of the first modification 

 alone are visible. 



But there is a circumstance relating to the formation of this 

 made that deserves attention. If it were, as it seems to be, a 

 dodecahedron with triangular faces, the two pyramids, of which 

 it v;ould be composed, being divided horizontally, would each 

 have for its base a regular hexahedral plane, divisible into six 

 equilateral triangles, fig. 197, and the six angles of the plane would 

 necessarily be 120° each. If a diagonal section of a crystal, 

 fig, 194, be made along the edges of the pyramids efa and c g h^ 

 and along those of the prism e e and a h, the plane given to each 

 portion by that section would also be a hexahedral plane, fig. 198. 

 But since it has been shewn that the two sections on fig. 194, 

 (represented by the lines a i c and e i >6, fig. 198) are parallel with 

 the edges (?yand g h, and y a and e g; and since the incidence 



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