46 Dr. T. Thomson's new Analysis of the 



paper in the Philosophical Magazine for May (vol. x. p. 373.) 

 is the same that had been previously read in Edinburgh. 



The analysis of a double carbonate of lime and bary tes is 

 very easy. Mine was made during the winter session, in a 

 laboratory where a number of practical students were con- 

 stantly engaged in experimenting. It was possible therefore, 

 as I stated to Mr. Johnston, that some accident vitiating my 

 results might have taken place without my knowledge. I de- 

 termined in the first place the constituents of the mineral, and 

 then ascertained the quantity of carbonate of barytes from the 

 nitrate left when the dry salt was digested in alcohol. Here 

 the analysis stopped. For I was not able for more than a 

 fortnight to analyse the nitrate of lime, and it was overturned 

 accidentally by one of the students. But from the known weight 

 of the mineral dissolved and the quantity of nitrate of barytes 

 obtained I inferred the composition of the mineral. 



I have now finished three successive analyses of this mine- 

 ral. For the greater security they were made in a separate 

 room which my practical students seldom entered, and I 

 watched over all the steps of the analysis in person. I shall 

 here state the steps of the process, that Mr. Johnston may re- 

 peat it if he should feel inclined. 



The first analysis was made upon 22 grains, the second on 

 20*1 grains, and the third on 15*8 grains of the mineral. It 

 will be sufficient if I give a detail of the third analysis, because 

 I consider it as the most accurate. 



1. The 15*8 grains of the mineral were put into a flask 

 along with dilute nitric acid, and digested on the sand-bath. 

 They dissolved with the exception of 033 grain of sulphate 

 of barytes. On testing the nitric acid employed I found in it 

 a sensible quantity of sulphuric acid. Hence I conclude that 

 this sulphuric acid was the cause of the appearance of the pre- 

 sent sulphate. In the first analysis the sulphate of barytes 

 undissolved weighed 0*2 grain, and in the second only 0'08 

 grain. These differences were doubtless connected with the 

 quantity of nitric acid employed to dissolve the mineral. 



2. The nitric acid solution was filtered, evaporated to dry- 

 ness, and redissolved in water. The solution was opal-co- 

 loured, and deposited when left at rest 0*27 grain of a white 

 matter, which proved when examined by the blowpipe to be 

 sulphate of barytes. This, added to the preceding quantity, 

 makes 0*6 grain of sulphate of barytes, equivalent to 0*5 grain 

 of carbonate of barytes. 



3. The aqueous solution of the nitrates was evaporated to 

 dryness, and the residual salt being put into a flask was di- 

 gested in a quantity of alcohol of the specific gravity O'BOO. 



