566 Mr. William Phillips on the Oxyd of Tin. 



bination with the planes of the primitive form, and with those of 

 the first and second modifications in fig. 174. I regret the not 

 having been able to ascertain the incidence of the planes of this on 

 those of any other modification. 



All the specimens on which I have noticed the planes of this mo- 

 dification, were long since brought from Cornwall, but from what 

 mine it is impossible now to ascertain. 



Twelfth Modification. 



The twelfth modification consists in a decrease on those edges of 

 the primitive crystal which are formed by the meeting of the two 

 pyramids base to base, by which each edge is replaced by two 

 planes, placed on the primitive faces, but inclining on the axis, 

 passing through the edges, fig. 182. PI. 23. By fig. 183, the 

 planes of this modification are represented in combination with the 

 secondary pyramid, as will be evident on consulting fig. 184, which, 

 together with fig. 185, represents the only crystals on which I have 

 noticed the planes of this modification. I have not been able to 

 ascertain their incidence on those of any other. 



Macks. 



Most of the crystals delineated in the series annexed to this paper, 

 are defined with great neatness and beauty : but there is generally 

 much seeming confusion among the crystals of the oxyd of tin, 

 arising principally from a circumstance or law, not altogether pe- 

 culiar to it, by which similar portions of two or more crystals are 

 regularly united, so as to form what have been termed macles^ one 

 of which has been described by De Lille by that name, and by Haiiy 



