Professor Apjohn on a new Variety of Alum. 159 



dissolving the alumina. In the prosecution, however, of the analysis, I became 

 convinced that this method of separation vpould not answer, for a trace of oxide 

 of manganese was dissolved by the caustic potash, and however great the excess 

 of alkali employed, and however long the digestion, a considerable quantity of 

 alumen remained with the oxide. 



To overcome this difficulty, a fresh portion of the mineral (30.4 grs.) was 

 dissolved in water, and precipitated by the yellow ferrocyanide of potassium. 

 The ferrocyanide of manganese thus thrown down, was collected on a single 

 filter, and being well washed, was transferred to a porcelain capsule, in which it 

 was digested with caustic potash. The oxide of manganese thus developed, 

 when collected, washed, and dried, at 212° on a double filter, was found to weigh 

 4.21 grains. Of these 3.87 were reduced by a white heat to 2.21. Hence, as 



3.87 : 2.21 : : 4.21 : 2.40 := red oxide of manganese, or that whose formula is 

 Mbj O4. And 



38.36 : 35,7 : : 2.4 : 2.23, the equivalent quantity of protoxide of manganese. 

 And 



30.4 : 2.23 : : 100 : 7.33, the protoxide of manganese in 100 grains of the 

 mineral. 



The washings of the ferrocyanide of manganese were now supersaturated with 

 sulphuric acid, and boiled with bicarbonate of soda added in excess, by which the 

 alumen was precipitated. When washed and dried at 212° it weighed 5.89 grs. 

 Of these 5.77 by a red heat were reduced to 3.18. Hence 



5.77 : 3.18 : : 5.89 : 3.24 = true weight of alumen. 



30.4 : 3.24 : : 100 : 10.65 = alumen in 100 grs. of the mineral. 



The following, therefore, are the results of the analysis : — 



Sulphuric acid .... 33.50— .71 = 32.79 



Alumina 10.65 



Protoxide of manganese 7.33 



Sulphate of magnesia 1.08 



Water of crystallization 48.15 



100.00 

 If these numbers be divided by the respective atomic weights, the following 

 will be the quotients : 



VOL. xviii. z 



