194 ANNUAL OF SCIENTIFIC DISCOVERY. 



which cobalt prevails, and which resist most perfectly the action of 

 water ; and of the more fusible silicates, in which potassa prevails, and 

 which are more susceptible of the action of water. The former sili- 

 cates constitute the azure or coarse blue, and the latter are partially 

 decomposed by water, which subtracts a sub-silicate of potassa and 

 leaves a supersilicate of potassa in a minutely divided state. Farbe 

 owes its tints to the subtraction* of potassa, and eschel contains more 

 silica and less potassa and cobalt than other varieties. The beauty of 

 smalt is said to be heightened by what may be called accidental causes 

 the presence of four or five per cent, of arsenic and arsenious acids 

 from six to nine per cent, of phosphoric, acid, and minute particles of 

 zinc, tin, antimony and nitre. On the other hand it is deteriorated by 

 the presence of nickel, lead, iron beyond ten per cent., bismuth, borax, 

 soda, the alkaline earths, alumina, felspar, fluorspar and sulphur. 



IMPROVEMENTS IN THE MANUFACTURE OF ALUM. 



MR. JAMES T. WILSON, of Middlesex, England, has recently patented a 

 new process of manufacturing alum, which consists in subjecting alumi- 

 nous shales to the direct action of a sufficient quantity of sulphuric 

 acid to saturate, at a single operation, all the alumina contained in 

 them, and convert it to the state of sulphate, the alum being obtained 

 by subsequent crystallization in the usual way. The shale should be 

 selected of a kind as free as possible from coal, or iron, lime, and other 

 soluble impurities, and, after having been exposed to the air two or 

 three months, to reduce it to small fragments, and then burned in a 

 lime or other similar kiln, is to be placed in an open boiler, about 15 

 feet long, 5 feet wide, and 4 feet deep, lined with lead, and having a 

 false bottom at a height of about 15 to 18 inches above the bottom, 

 composed of lead of about 24 Ibs. to the square foot, perforated with 

 numerous holes of half an inch diameter, and supported on an iron 

 framework, and provided with suitable conveniences for allowing the 

 bottom of the boiler underneath to be examined and cleaned out. Into 

 this boiler, which is to be placed over a furnace, in such a position that 

 the flame does not rise to within a couple of inches of the false bottom, 

 the shale is then deposited, the larger pieces being screened out, and 

 laid immediately on the false bottom, and sulphuric acid of a specific 

 gravity of 1.845, added in the proportion of 10 cwts. to every 12 cwts. 

 of shale, with sufficient water to reduce the specific gravity to 1.35 or 

 1.4, and nearly fill the boiler. Heat is applied, and a gentle ebullition 

 kept up for eight-and-forty hours, when the whole of the available 

 alumina will have been dissolved, and the solution will be fit to be 

 treated for crystallization, by using sulphate of potash or ammonia. 

 The patentee has, however, observed, that, when alum is at once crys- 

 tallized from solutions obtained as above described, a certain quantity 

 of the acid in such solution remains in excess, and renders the purifi- 

 cation of the alum a matter of difficulty. Now, to obviate this objec- 

 tion, he introduces into the solution either ammoniacal liquors of gas 

 works, or condenses therein vapors containing ammonia which com- 

 bines with the excess of acid, and renders the solution fit for immediate 

 crystallization. 



