452 Mr. Levy on Murchisonite. 



gical species which appeal's most consistent with the actual state 

 of the science is, that a mineral species contains all the indivi- 

 duals composed of the same principles united in the same 

 proportion, and when regularly crystallized, referable to the 

 same primitive form. Now since cleavage has in every in- 

 stance observed some very simple relation with the dimensions 

 of the primitive, if we meet with a substance having a great 

 resemblance to another, but having a cleavage that does not 

 correspond to some simple modification of the primitive form 

 of that other substance, — we must necessarily infer, that the 

 first has a primitive form differing at least in its dimensions 

 from the primitive form of the second, and consequently, ac- 

 coi'ding to our definition of the species, must constitute a new 

 one. To the essential difference existing between the new 

 substance and felspar, it may be added, that the first has no 

 cleavage parallel to the lateral planes of the primitive form of 

 the second, which most of the varieties of the latter present. 

 But this difference would not be sufficient, since the facility of 

 cleavage seems to vary with circumstances. Thus Mr. Fara- 

 day has discovered the means of obtaining crystals of sul- 

 phate of copper in which he may increase at will the facility 

 of cleavage parallel to one of the primitive planes of that sub- 

 stance ; so that he can even make it to crystallize in a mica- 

 like state with a nacreous reflection of light on the face of the 

 easiest cleavage. 



I shall propose for the substance I have described the name 

 of Murchisonite, in compliment to the gentleman who first di- 

 rected my attention to it, and whose zeal for mineralogical 

 science is so well known. 

 Fiff. 1. 



LXXXVIII. Proceedings of Learned Societies. 



ROYAL SOCIETY. 



April 5. — T^HE reading of Dr. Thomson's paper, On the com- 

 -■- pounds of chromium, was resumed and concluded. 

 The principal object of this paper is to give an account of a singu- 

 lar 



