18 Dr. Thomson's Anali/sis of [Jan. 



twenty years have elapsed without any thing having been added 

 to our knowledge of this ore, I was induced to attempt the 

 analysis of it on the supposition that it was likely to throw some 

 light upon the atomic weight of osmium, which I had tried in 

 vain to determine by other methods. 



After some preliminary experiments, I found the most success- 

 ful method of proceeding was to heat the ore with a mixture of 

 caustic potash and nitre. A platinum crucible could not be 

 employed, because at the heat which I found it necessary to 

 use, the crucible was sensibly corroded. I tried green glass 

 vessels with no better success ; and at last made choice of a 

 silver crucible, which, when the quantity of saltpetre was not 

 too great, was not sensibly acted on at a red heat. 



Ten grains of very pure plates of ore of iridium were heated 

 in a silver crucible over a lamp in a mixture consisting of about 

 50 grains of potash and 20 of saltpetre, and kept for about 

 half an hour in a heat approaching to redness, and ultimately 

 quite red. The potash and nitre soon fused, and became quite 

 black and opaque. When all action seemed at an end, the 

 process was stopped, water was poured into the crucible, and. 

 the whole potash and nitre washed out. An opaque liquid was 

 thus obtained, which, when left for 24 hours, deposited a number 

 of black flocks. The residual hquid had an olive-green colour. 

 When heated, it deposited some more black flocks, and became 

 colourless. It had a strong smell of osmium. 



The residual ore of iridium was black on the surface ; but 

 being digested for 24 hours in muriatic acid, it assumed the 

 metalhc lustre as at first. The muriatic acid liquid remained 

 transparent; but had acquired a reddish brown colour. The 

 ore of iridium by these two processes was reduced to 7-35 grs. 



Being again heated with potash and nitre, and afterwards 

 digested in muriatic acid, it was reduced to 6"37 grains. 



After being subjected a third time to this treatment, it was 

 reduced to 4'46 grains. 



A fourth heating with potash and digestion in muriatic acid 

 reduced it to 2*78 grains. 



A fifth to 2"3 grains. 



A sixth to 1 '5 grain. 



A seventh to TIS grain. 



This small residue had the metallic lustre, and exactly the 

 characters of the original one. I, therefore', stopped the process 

 here. If we subtract the ]*18 grain of residue from the 10 

 grains taken, there will remain 8*82 grains for the quantity 

 actually subjected to analysis. 



All the potash solutions were collected together and boiled in 

 a flask till they became limpid and colourless. Osmium was 

 given out, and a black flocky precipitate fell, which, being 

 collected and dried on the filter, weighed o'4 grains. 



