Pirsson and Wells — Occurrence of Leadhillite. 225 



It will be shown beyond that Groth's formula is the correct 

 one, and it is evident that the earlier analysts would have 

 arrived at it if they had detected and determined the water. 



Chemical Analysis. — Some of the methods that have beeu 

 previously used for the analysis of leadhillite have been rather 

 severely criticized, and the operation has been characterized as 

 attended with difficulties. It seems necessary, therefore, to 

 give the process used in the present investigation with some 

 detail. 



The carefully selected material was pulverized, and about 

 2 g of the substance, without any artificial drying, was weighed 

 in a platinum boat. The boat was placed in a. dry piece of 

 combustion tubing, to one end of which the ordinary appara- 

 tus for weighing water and carbonic acid was attached. A 

 slow stream of pure, diy air was passed through the tube and 

 apparatus, and the substance was heated to low redness until 

 the water and carbonic acid were driven off and absorbed in 

 the weighed apparatus. The sum of the weights of the water 

 and carbonic acid varied but 0*07 per cent from the loss in 

 weight of the substance. The water in the calcium chloride 

 tube was neutral to litmus paper, thus showing that no S0 3 

 had been driven off. In fact a much higher temperature than 

 that used will not decompose lead sulphate if reducing gases 

 are absent.* 



The ignited substance was transferred to a platinum crucible 

 and fused with three or four parts of sodium and potassium 

 carbonates to which a trace of potassium nitrate was added. 

 The mass was treated with 2 or 300 cc of hot water to which 

 a little ammonium carbonate was finally added. After cool- 

 ing, the residue was filtered off and washed. Preliminary ex- 

 periments had shown that the filtrate from this contained 

 scarcely a trace of lead, and it was therefore used for the deter- 

 mination of sulphuric acid, as barium sulphate, in the usual 

 way. 



The insoluble residue, consisting of lead oxide with some 

 carbonate, was dissolved in dilute nitric acid. The solution 

 was complete except that a* trace of lead peroxide remained 

 which was dissolved in a little hydrochloric acid. The solution 

 containing all the lead was evaporated with sulphuric acid, and 

 after taking up the residue with water, the lead sulphate was 

 collected in a G-ooch crucible, ignited to low redness with care- 

 ful protection from the action of reducing gases and weighed. 

 ^Nothing could be found in the filtrate from the lead sulphate 

 except extremely minute traces, probably of lead and iron. 



* Fresenius states that lead sulphate does not lose weight at the most intense 

 redness. (Quant. Anal , Analyt. Expt. No. 52.) 



