158 Professor Apjohn on q, new Variety of Alum. 



conclusion, It was necessary first to investigate the proportions of the three con- 

 stituents just mentioned; and, secondly, to determine whether the alkali was or 

 was not altogether absent. 



To resolve the latter question, 30.13 grains were dissolved in water, and 

 treated first with ammonia, which threw down the alumen and greater part of the 

 manganese, and then with sulphuretted hydrogen, to complete the precipitation 

 of the latter base. The whole was then thrown upon a filter, and the mixed 

 precipitates being well edulcorated with distilled water, the washings were evapo- 

 rated to dryness in a porcelain capsule, and then transferred to a counterpoised 

 platinum crucible, in which they were ignited. The residue weighed but .31 of 

 a grain, and upon examination proved to be sulphate of magnesia, with a scarcely 

 appreciable trace of sulphate of lime, in all probability derived from the filter. 

 Hence, as 



30.13 : ,31 : : 100 : 1.02, the amount of sulphate of magnesia in 100 grains, 

 — and which, as may be easily calculated, includes .71 of a grain of sulphuric 

 acid. 



To determine the proportions of acid, alumen, and manganese, 34.47 grains 

 were dissolved in hot water, and nitrate of barytes being added in excess, the pre- 

 cipitate was collected, and well washed with distilled water upon a double filter. 

 Dried at 212° the sulphate of barytes weighed 35.26 grains. Of these 31.13 

 were exposed to a low red heat, by which they were reduced to 29.71- Hence, 

 as 



31.13: 29.71:: 35.26: 33.65, the true weight of the sulphate of barytes. 

 And as 



116.8 : 40.1 :: 33.65 : 11.5.5, the corresponding sulphuric acid. And as 



34.47 : 11.55 : : 100 : 33.50, the sulphuric acid in 100 grains of the mineral. 



To the washings of the sulphate of barytes, sulphuric acid was added, so as 

 to precipitate the excess of the barytes, and this being separated by a single 

 filter, ammonia was first added in excess, and then a current of sulphuretted 

 hydrogen directed into the solution, by which operations the alumen was thrown 

 down, and the manganese also, partly as oxide, and partly as sulphuret. The 

 mixed precipitate was well washed upon a single filter, and then transferred to a 

 porcelain capsule, where it was digested with caustic potash, with the view of 



