Prof. Johnston on Baryto-calcite 373 



For the present let it suffice to have indicated the general 

 causes of the radiated appearance seen in Mr. MacVicar's ex- 

 periment. J. T. Graves. 



[Communications on the same subject have been received 

 from Mr. H. S. Peacock of London, Mr. William George 

 Horner of Bath, Mr. Robert Wilson of Glasgow, Mr. J. l5e 

 C. Sowerby of Camden Town, Mr. G. Dodd of London, and 

 Mr. G, H. Hoffman, Surgeon, of Margate. They all agree 

 in referring the phaenomenon in question to the principles of 

 reflection and perspective explained in the preceding paper, 

 and the three latter gentlemen respectively mention that they 

 had observed the radiated appearance for considerable periods 

 before the publication of Mr. Mac Vicar's letter. — Edit.] 



LXXVL On the Composition of the Right Rhombic Baryto- 

 Calcite^ the Bicalcareo- Carbonate of Baryta o/'Dr.l'homson. 

 By James F. W. Johnston, A,M.^ F.R.S.E.^^ Professor of 

 Chemistry and Mineralogy in the University of Durham,* 



TN the sixth volume of this Journal, p. 1, 1 described a 

 -*- mineral occurring in the lead-mine of Fallowfield near 

 Hexham, Northumberland, which I had found on analysis to 

 be a true baryto-calcite, though having a form wholly irrecon- 

 cileable with the doubly oblique prism of the original baryto- 

 calcite of Brooke. 



Since the publication of this description, Dr. Thomson of 

 Glasgow inserted in the Records of Science, vol. i. p. 369, 

 and more recently in his System of Mineralogy, vol. i. p. 141, 

 an account of a mineral from Bromley Hill mine near Alston, 

 having the same specific gravity and the same crystalline form 

 as my new baryto-calcite, but having a different composition. 



My attention was first drawn to this circumstance b}' my 

 friend Mr. Brooke, who at once recognised my second form 

 of baryto-calcite in the mineral of Dr. Thomson, and aware 

 of the importance of a knowledge of the true composition of 

 this mineral to the doctrine of isomorphism, requested to know 

 whether I had actually analysed my specimens from Fallowfield. 



My analysis gave the formula Ca C-f-Ba C, that of Dr. Thom- 

 son 2 Ca C + Ba C, and hence the name Bicalcareo-carbonate 

 of Baryta which he has assigned to the mineral. 



Unwilling to leave the matter in doubt I repeated my ana- 

 lysis of the Fallowfield mineral. 



* Communicated by the Author. 



