1835.] Improvements in Science." 269 



form between them an angle of 108° *26; the third is per- 

 pendicular to the axes of the same octahedron, and lead to 

 a rhomboidal prism under an angle of 108° 26'. Junckerite 

 possesses a yellowish gray colour, very similar to certain 

 varieties of Scheelin. Before the blow-pipe with borax it 

 forms a yellowish, transparent, green glass. Sp. gr. 3*815. 

 It was found in the mine of Poullaouen, (Finistere) in small 

 quartz veins, which traverse the greywacke in which the 

 mine exists. The name was applied in honour of M . Juncker 

 the director, by M. Pailette sub-director, by whom the 

 mineral was discovered. It consists of 



Protoxide of iron . . 53*6 



Carbonic acid . . . 33*5 



Silica 8*1 



Magnesia 3*7 



Loss 1*1 



100-00 



Most of the carbonates crystallize in rhombohedrons. 

 Those which do not, such as the carbonates of barytes, 

 strontian, lead, &c, possess a crystalline form analogous to 

 arragonite. Analogy would, therefore, induce us to believe, 

 that we know only one of the forms of these carbonates, 

 and that if they were met with in another form, that form 

 would be a rhombohedron. Junckerite presents a second 

 example of a carbonate, occurring in the rhombohedral 

 form, and in that of a right rectangular prism. The car- 

 bonate of lead crystallizes in the form of a right rhomboidal 

 prism, under angle of 117°, which differs from arragonite by 

 only 50 or 55 minutes ; but the sulpho-carbonate of lead, 

 from Leadhills, described by Mr. Brooke, occurs in rhom- 

 bohedrons with an angle of 107° 30'. 



Dufrenoy considers that this combination is not a dis- 

 tinct substance, but only a carbonate of lead mixed with 

 sulphate of lead,* because the two elements are not in de- 

 finite proportions, and because it would present the third 

 instance of a dimorphous carbonate, and we should then 

 have a similar relation between the angles of the rhombo- 

 hedral carbonates 105° 5', 107°, 107° 30', as with those of the 



