18 H.] On the Composition of Arragonite. 245 



stant proportion. It must therefore be considered as a triple 

 compound of carbonic acid with lime and strontian The propor- 

 tion in which carbonate of strontian exists in the arragonite is 

 between 3 and 4 per cent. 



Tbat so considerable a proportion of carbonate of strontian in this 

 mineral could have been overlooked by those who hitherto examined 

 it, must I think be ascribed to this circumstance, that they con- 

 ceived sulphate of strontian to be as insoluble in water as sulphate 

 of barytes ; and therefore supposed, that if arragonite contained 

 strontian, that earth would be precipitated from the muriate or 

 nitrate by means of sulphuric acid, or a sulphate, when so much 

 diluted with water that the lime would remain in solution. But 

 that this supposition is not accurate, I have already had an oppor- 

 tunity of showing, in my analysis of sulphate of strontian from 

 Suntel. 



Besides, strontian and lime have many common chemical proper- 

 ties. Hence there is some difficulty in separating them from each 

 other; and I only succeeded in my object by dissolving arragonite 

 in pure nitric acid, evaporating the solution till it crystallizes, and 

 digesting the crystals in alcohol, which does not dissolve the nitrate 

 of strontian. 



It is easy to satisfy oneself that arragonite contains strontian, by 

 evaporating a solution of it in nitric acid. When it is sufficiently 

 concentrated, crystals of nitrate of strontian form in the cold solu- 

 tion; and even sometimes during the evaporation, when the solu- 

 tion is very neutral, small octahedral crystals of nitrate of strontian 

 fall down, which remain undissolved when washed in alcohol. 



But is this admixture of carbonate of strontian sufficient to ac- 

 count for the striking difference in structure between arragonite 

 and calcareous spar ? This is a question of some intricacy ; but I 

 conceive it must be answered in the affirmative. Various experi- 

 ments which I have made on native specimens of bitterspar* 

 appear to me to prove decisively, that a small quantity of one sub- 

 stance, mixed with a great quantity of another, may notwithstand- 

 ing determine the figure of its crystals. Thus, I have analysed per- 

 fect rhombs of bitterspar, from St. Gothardt, which contained only 

 7 per cent, of carbonate of lime. It seemed likewise very evident, 

 from the analysis of various specimens of sparry ironstone, or sleel- 

 itime, as it is called, that their rhomboidal form is owing to the 

 carbonate of lime which they contain. Is it not therefore probable 

 thai the crystalline form of arragonite b owing to the carbonate of 

 •Croatian mixed with the carbonate of lime ? As the carbonate of 

 strontian has not hitherto been found fully crystallized, its structure 

 is unknown ; so that we are unable to recognize it in arragonite. 



• This id iif .•xperimrnls wai undertaken in order to examine the law Riven by 

 Berxdloi in the \nnalender Phyiik, xl. .W5, for the formation of triple alu (or 

 ionble -.ills, u ii<- calli them). Tin- retaki which 1 obtained accord perfectly with 

 tlic rlewi of that philosopher, and nerve to confirm tbem. 



