190 



on the edges of the original six-sided column, forming six- 

 sided faces : see the dotted lines on the column of the right 

 hand figure, and also the apex which is terminated by six 

 trapezoidal faces. Thus we should have 48 faces if they 

 were regular; this is certainly a curious modification. Mr. 

 Laing judiciously observed, that the sulphuret of lead, or 

 galaena, in most cases, where it is decomposing to form 

 carbonate of lead, has a blue tarnish. It sometimes also 

 becomes dusty or crumbly. 



