256 Mr. J. Brown on the Molyhdate of Lead, 



Oxide of lead . . 59'0 58*0 \ corrected from the 



Molybdic acid . . 40*5 40*5 J chloride. 



99-5 98-5 



This method is essentially the same as that used by Klap- 

 roth. The result however is much nearer the truth. Gobel 

 however gets too much molybdic acid and too little oxide of 

 lead. This was probably owing to some of the chloride of lead 

 not being obtained, as it is soluble to a great extent in water 

 (1 in 152 of water)*; and the analyst does not state how he 

 washed the chloride of lead free from molybdic acid. 



IV. The methods hitherto employed being liable to very 

 great objections, the molybdate of lead was analysed by another 

 method, which had proved successful in the hands of Mr. 

 William Parry last year in the Glasgow College laboratory. 



26*84 grs. of the mineral finely pounded were boiled for a 

 considerable time with nitric acid and filtered. The unde- 

 composed mineral, along with a quantity of molybdic acid, 

 remained on the filter. This was then completely washed : 

 ammonia was then poured into the filter. The molybdic acid 

 was thus dissolved, and the insoluble matter remained on 

 the filter ; this was then washed, dried, ignited and weighed. 

 The weight of the insoluble matter in 26*84 grs. was 1*15 gr. 



The solution containing the molybdate of ammonia was 

 then evaporated to dryness, and heated to redness in a close 

 vessel. The greater part of the molybdic acid was thus ob- 

 tained. Its weight was 6*76 grs. 



The first washings from the molybdic acid and the insoluble 

 matter were then concentrated. Caustic ammonia was added 

 in order to neutralize the excess of acid, and afterwards sul- 

 phohydret of ammonia was added in excess. In this manner 

 the lead was precipitated in the form of sulphuret, while the 

 tersulphuret of molybdenum was redissolved in excess, giving 

 the solution a deep red colour. The sulphuret of lead was 

 then thrown on a filter, and washed with water containing 

 sulphohydret of ammonia. When completely washed, the 

 sulphuret of lead was dissolved in muriatic acid, and after 

 boiling for some time was filtered to get rid of the sulphur. 

 The filtered liquor was then concentrated, and the lead pre- 

 cipitated by means of o:falate of ammonia. The precipitated 

 oxalate of lead was then thrown on a filter, washed and dried. 

 By ignition the oxalate of lead was converted into the oxide ; 



* I found in two experiments that 3963 grs. of water at 60° dissolved 

 26-2 grs. Pb CI = 1 in 151, and 4260 grs. HO dissolved 27-6 grs. Pb CI = 1 

 in 154 HO. 



