Mr, William Phillips on the Oayd of Tin. 369 



angle, described by the dotted lines, must exist between the planes 

 2, 2, of each, which are those of the second modification ; in- 

 stead of which the whole space between those planes is occupied 

 by an elongation proceeding from each, so as to connect and form 

 the two upper planes, 2, 2, into one plane ; the same effect takes 

 place in regard to the two lower , 



The series of this double made is given by figs. 216 and 221, 

 PI. 25, placed in the position in which they are most commonly 

 found : they usually present but one termination, the other being 

 imbedded in the matrix. Fig. 217, represents a crystal similar to 

 that of fig. 190, but with a shorter prism, so placed as to shew 

 most advantageously the section described on fig. 190, and 

 thereby serve as a clue to the more ready comprehension of the 

 series. On each figure the planes of the several modifications are 

 pointed out, by the number of the modification itself being placed 

 on them. On figs. 218, 219, and 221, the planes of the primitive 

 form are visible. These macles are generally defined with great 

 neatness, and mostly allow of the perfect use of the reflecting 

 goniometer, which has been employed to corroborate what has 

 been said of their construction, the truth of which it places be- 

 yond a doubt. I possess macles represented by figs. 218 and 220, 

 on which both terminations are complete. 



Incidence of 4 on 4, on the summit of fig. 218 — 112°. 10'. 



This made seems to verify the conclusion of Lhermina, that the 

 section a b c d^ fig. 190, takes place parallel with the edges ^y and 

 g /&, which are those of the secondary pyramid. If the termina- 

 tions i^ ^ of fig. 187, were complete, or, in other words, if the 

 planes of the second modification were not visible, fig. 187 would 

 take the form of fig. 192. Of fig. 192, let a section along the 



Vol. 11. 3 a 



