242 Mr. Levy on a new Mineral Substance. [Oct. 



measured are the incidences of e* and a 1 on the base, and it is 

 from these data, and from the supposition that 4 and 1 are the 

 indices of these planes, that the angles and dimensions of the 

 primitive are deduced. I have thus found that the lateral planes 

 of the primitive were inclined to each other at an angle of 

 114° 20', and that the height was to one side of the base nearly 

 in the ratio of 25 to 12. The other angles are (a',p) = 104° 75', 

 (e» = 148° 30'. 



It may appear strange that in the want of sufficient data to 

 determine the primitive form, and being obliged to make a sup- 

 position upon the laws of decrements which produce the faces 

 e 4 and a 1 , I have not chosen the simpler hypothesis of each of 

 these faces being the result of a decrement by one row. In that 

 case the lateral planes of the primitive would have been inclined 

 at an angle of 162° 18', and had I supposed 2 instead of 4 for 

 the index of the face e 4 , the incidence of the lateral planes 

 would have been 145° 25'. Kow though I could not measure 

 the angle of the planes M, fig. 7, their incidence appeared to me 

 much nearer to the angle 1 14° 20', which I have chosen, than 

 any of the other two very obtuse angles ; this circumstance 

 added to some indications of cleavage in the direction of the 

 same planes, determined me to adopt the number 4. 



The crystals are placed upon mamillated green carbonate of 

 copper lying upon massive red copper. 



Upon a specimen ofwavellite, from Cornwall, belonging to 

 the same collection, I have observed some minute white trans- 

 parent crystals in the form of acute rhombic octahedrons, with 

 their summits replaced by a plane, see fig. 8. This form is not 

 incompatible with wavellite, whose primitive form is a right 

 rhombic prism. However, in trying to split some of the crys- 

 tals, I could not perceive any indication of the cleavages which 

 exist in wavellite. The only means to ascertain whether their 

 form could be derived from that of wavellite was in the follow- 

 ing manner. First, it is obvious that, in that case, one of the 

 parallelograms A BCD, ABCE, BEDF, must be parallel to 

 the base of the primitive ofwavellite. Secondly, one of them 

 must be similar to that base, or at least must be such that when 

 placed in the plane of that base so that its diagonals be parallel 

 to the diagonals of the base, the sides must be found parallel to 

 lines drawn from one of the angles of the base to some simple 

 multiple or part of the opposite sides. If none of these condi- 

 tions be satisfied, then it may be safely inferred that the two 

 forms are incompatible. But the application of this method 

 supposes that the incidences of the faces of the crystals can be 

 measured with great accuracy, and here the planes were not 

 sufficiently brilliant to answer in the measurements of less than 

 half a degree. Dr. Wollaston kindly undertook an examination 

 of this substance, and the results of his observations were, as 





