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IX. On the Right Rhombic Baryto-Calcite, with reference 

 to Prof. Johnston's Paper in the Phil. Mag. for May 1837. 

 By Thomas Thomson, M.D., F.R.S. L. $ E., Regius 

 Professor of Chemistry, Glasgow. 



To the Editors of the Philosophical Magazine and Journal. 

 Gentlemen, 



'T^HE Number of the Philosophical Magazine for May, which 

 -■■ I received yesterday, contains a paper by Mr. Johnston 

 of Durham, entitled " On the composition of the right rhom- 

 bic Baryto-Calcite, the Bicalcareo-carbonate of Baryta of Dr. 

 Thomson." This paper makes it necessary for me to send you 

 a few observations, and to request their insertion in the next 

 Number of the Philosophical Magazine. 



The mineral which constitutes the subject of Mr. Johnston's 

 paper came into my hands in the year 1834, being in a collec- 

 tion exposed here for sale by a mineral dealer from Alston. 

 It was new to me ; I therefore analysed it, and concluded from 

 my experiments that it was a compound of 1 atom of carbonate 

 of barytes and 2 atoms of carbonate of lime. I was not aware 

 when I published an account of this mineral in May 1835 that 

 Mr. Johnston had some months before noticed it in a paper 

 containing some ingenious speculations about isomorphism 

 (L. & E. Phil. Mag., vol. vi. p. 1.), till my attention was called 

 to his paper by my nephew, Dr. R. D. Thomson of London. 

 I had read the paper before my System of Mineralogy went to 

 press. But as the result of Mr. Johnston's analysis was different 

 from mine, and as my faith in the theory of isomorphism and 

 dimorphism was not very strong, I thought the best thing I 

 could do was not to refer to Mr. Johnston at all, as I had not 

 complete evidence that his mineral was the same as mine. 



Last winter I received a letter from Mr. Johnston informing 

 me that he had repeated his analysis and found it correct, and 

 requesting me to repeat mine. At that period of our session 

 my time was so fully occupied with college business that it was 

 impossible to pay the requisite attention to the necessary steps 

 of an analysis. I therefore wrote him that at present I could 

 not spare time for experimenting, but that I would make a 

 point of repeating the analysis as soon as our session was over. 



Accordingly, when the month of May arrived, I immediately 

 set about it. While engaged in it I was informed by a friend 

 that a paper by Mr. Johnston had been read to the Royal 

 Society of Edinburgh, in which he maintained the accuracy of 

 his own analysis and the inaccuracy of mine. I presume the 



