THE 



LONDON AND EDINBURGH 



PHILOSOPHICAL MAGAZINE 



AND 



JOURNAL OF SCIENCE. 



[THIRD SERIES.] 



JANUARY 1835. 



I. On the Dimorphism of Baryto-calcite. By James F, W. 

 Johnston, Esq., M.A, F.R.S.Ed., F.G,S., Sfc, Reader in 

 Chemistry and Mineralogy in the University of Durham,^ 



nPHE substance first examined crystallographically and de- 

 -*- scribed as a new mineral species by Mr. Brooke, under 

 the name of Baryto-calcite, is now well known to mineralogists, 

 and is to be ipet with in most cabinets. It has a specific gra- 

 vity of 3°*66, according to Mr. Children, and its form, by the 

 measurements of Mr. Brooke, is an oblique rhombic prism 

 M on M = 106° 51', M on P 102° 54'. 



Since the discovery of the principle of dimorphism, more 

 particularly since the analysis of the plumbo-calcite enabled 

 me to class the carbonate of lead with that of lime as isodi- 

 morphous bodies, I have looked to this mineral with very 

 considerable interest. If lime, barytes, strontian, and prot- 

 oxide of lead constitute an isomorphous group, of which 

 two are already observed to be c^zmorphous, we may naturally 

 look for a similar property in the other two ; we may expect 

 the carbonates, for example, of barytes and strontian, either 

 in their pure state, or in combination with another carbonate 

 belonging to the same group, to crystallize in two or more in- 

 compatible forms. But the baryto-calcite measured by Mr. 

 Brooke gave us none of that intbrmation we expected from 

 it. The crystalline form was neither that of the carbonate of 

 barytes in its usual form, nor that of the carbonate of lime in 

 calc spar ; it was neither a rhomboid nor a right rhombic 



* Communicated by the Author. 

 Third Series. Vol. 6. No. 31. Jan. 1835. B 



