48 On a native Combination of Sulphate of Barium [J v l y, 



Article III. 



Ah Account of a native Combination of Sulphate of Barium and 

 Fluoride of Catcium. By James Sraithson, Esq. F.R.S. 



(To Dr. Thomson.) 



SIR, Paris, March 24, 1820, 



I ACQUIRED this substance in Derbyshire. It is many years 

 since I ascertained its constitution. I have examined several 

 minerals which in appearance bore a resemblance to it, but have 

 not found any of them to be of the same nature. This species 

 would hence appear to be of rare occurrence in the earth. 



This substance formed a vein about an inch wide in a coarse 

 shell limestone. Next to this substance \a as a layer of crystals 

 of sulphuret of lead ; and between these and the limestone rock 

 a layer of crystals of carbonate of calcium. 



I infer that these matters filled a vertical fissure in the lime- 

 stone stratum ; and from the ideas I entertain of the mode by 

 which such fissures have generally become occupied by their 

 contents, 1 beheve them to have been successively deposited in 

 it by sublimation, either through the intense vehemence of sub- 

 terranean fire, or by the agency of the vapour of water, or of 

 some other gas. 



This compound matter bears in its general appearance so 

 strong a resemblance to fine compact greyhmestone that the eye 

 can probably not distinguish between them. 



Forty-two grains of it lost 11-2 grs. in rain water at the tem- 

 perature of 61° Fahr. ; consequently its density is 3-750. 



These 42 grs. of this stone by laying in the water did not 

 absorb into their substance a quantity of it equal to one-tenth of 

 a grain. 



It does not mark glass, and is readily scraped to a powder by 

 a knife. It marked sulphate of barium. Its hardness and that 

 of fluoride of calcium appeared to be the same. 



It showed no electricity by heat. By friction it readily 

 became electrified. 



In the fire it lost no weight. 



At the blow-pipe, it readily melted. The little bead while in 

 fusion was transparent. On evolving, it became opaque. The 

 transparency of the bead in a melted state is best seen with a 

 very minute one. On fusing this matter long, it spreads on the 

 coal, and becomes a refractory mass. 



With borax, it dissolved with great effervescence into a brown 

 glass. If much stone was used, the glass appeared quite black, 

 but drawn out to a thread with the tongs, it was found to be of 

 a fine hyacinth colour. These colours depend on the formation 

 of s»ilphur. 



With microcosraic salt it fused with effervescence to a clear 



