tKHi, 



■5^4 SULJPHUftET OF L£AI>, COPPER, AND ANTIMOK1T. 



©n the longitudinal edges of the primitive prism ; in fig. 5 # 

 those I have observed along the edges of the terminal faces ; 

 and in fig. G, those that I have observed at the angles of 

 these faces. These three figures are intended for the t?ame 

 purpose of convenience, as those which, in my former paper 

 on this substance*, were given solely with this view, and the 

 exact models of which had not yet been observed in nature. 

 My experience in crystallography has frequently led me to 

 remark, that, when the crystals of a substance are liable to 

 any considerable number of modification*!, and at the same 

 time actually undergo seveial of them, this method is ex- 

 tremely useful, and frees the mineralogist, who is desirous 

 of ascertaining one of these crystals, from a task not unfre- 

 quently very troublesome. There are even substances, in 

 which this method is very advantageous to the most expert 

 crystallographer, and the present is one of them. 

 F*t modified- \Jl modification-. The planes arising from this modifier 

 tion substitute for the longitudinal edge« of the primitiye 

 prism a plane equally inclined to those contiguous to it. 

 They are produced by the rctrogradation of one row of the 

 particles of the laminae along these edges. These new 

 planes are frequently striated, as is shown in fig. 2. Some- 

 times they cause the complete disappearance of the faces of 

 the primitive crystal, giving rise to another prism, which 19 

 likewise a rectangular tetraedron with square bases, but se- 

 condary to the primitive prism; aud in the crystals of this 

 variety that I have seen the faces were constantly striated, 

 as in fig. 3. This variety even led me into a mistake, when 

 I wrote the first paper on this substance I presented to the 

 Royal Society, by inducing me to consider the planes of the 

 prism of the varieties represented at figs. 10, 11, 15, iG, and 

 17, as belonging to tin ra : but a more attentive examination, 

 elucidated by observation since made on a great number 

 of other crystals, has taught me, that these striae were the 

 simple effect of aggregation, and that these same planes be- 

 longed to then pj unitivtsf. 



I shall 



* JiJ( y i:innal »o.'e, — — and for which 1 was so unhandsomely re- 

 proved by Mr. Smith* on, in his critique printed in the Philosophical 

 fransactioris. 



■f The striae, that cccur so frequently on the planes of crystal;, are 



otten 



