36 Dr. Clarke on a new Variety of Green Fluor Spar. [July, 



of this remarkable variety of the Jiuate of lime adds greatly to 

 the peculiar play of light and beauty of the specimens. 



Until these crystals were sent to Cambridge I had never seen 

 crystallized^/««£e of lime in the form of parallelopipeds, neither 

 does Mr. Jameson, in his valuable work on mineralogy, mention 

 any such fonn ; * their major diameter is to their minor diameter 

 as three to two. The other forms exhibited by the same //mo/- is 

 the cubic, with or without bevelled edges. The miners are 

 assured of the neighbourhood of cavities lined with crystals by 

 the increasing hardness of the substance of the vein. 



This kind ofjiuor is also remarkably distinguished from every 

 other variety by its phosphorescence ; which is such that it becomes 

 visible in a dark room simply by placing a large crystal in water 

 heated nearly to the boiling temperature. But to exhibit its 

 phosphorescence in perfection, it is necessary to reduce the mineral 

 to powder in a mortar, and then scatter its particles upon the 

 surface of an iron plate, heated nearly to redness. It then 

 phosphoresces with a violet-coloured light. Mineralogists who 

 have added to the nomenclature of the varieties oi'jiuor by the 

 introduction of the word chlorophaue (as applied to the substance 

 which emits a green light when heated), may, if they choose, 

 call the Durham Jinor by the name of cyanophatie. Its other 

 characters are those which are common to every variety of the 

 crystallized ////ate of lime. Its specific gravity, estimated in dis- 

 tilled water at a temperature of 65 of Fahrenheit , equals 3-14. 

 Before the blow-pipe, when supported on platinum foil, it decre- 

 pitates, beautifully phosphoresces, loses its colour, becomes 

 highly limpid, and is ultimately reduced to an opaque white 

 enamel. Upon charcoal, its fusion is more readily accomplished ; 

 the results being the same. When first brought from the mine, 

 it is extremely fragile ; but (notwithstanding its natural brittle- 

 ness and inferior hardness) when properly desiccated, owing to 

 the intensity of its fine green colour, it is probable that lapidaries 

 will cut and sell its transparent crystals as spurious imitations of 

 tuclase or emerald. I remain, Sir, faithfully yours, 



Cambridge, May 1 0, 1 8 1 9. EliWAE'D DANIEL CLARKE. 



P. S. The crystals sometimes form two parallelopipeds, having 

 a common solid angle ; in this case the twin crystal is found at 

 the re-entrant angle, and its superior edge declines towards the 

 common angle ; although not always directly. Some rough 

 crystals, which are not highly transparent, are marked with 

 small cavities, in the form of the inferior pyramid of a regular 

 octahedron. 



* See Jameson's Mineralogy, vol. ii. p. 22 c 2. Edinb. 1816. Amon^ the remark- 

 able instances of crystallization exhibited by the filiate of lime, there is one 

 preserved in the Wcodwardian Collection at Cambridge, exhibiting an octahedron 

 formed by the juxta-positiou of cubes. I possess crystals of Siberian fluor with 26 



