122 ALUM, FEATHER 



during six or eight weeks. At the end of this time, it must be -washed, to extract the 

 sulphate of alumina. With this view, it may be treated like the roasted alum ores 

 previously described. If potash-alum is to bo formed, this sulphate of alumina is 

 evaporated to the specific gravity of T38 ; but if ammonia-alum, to the specific gravity 

 of only 1-2-t ; because the sulphate of ammonia, being soluble in twice its weight of 

 water, will cause a precipitation of pulverulent alum from a weaker solution of sul- 

 phate of alumina than the less soluble sulphate of potash could do. 



In preparing alum from clay or shale, it is of infinite importance that so much and 

 no more heat be applied to the clay or shale, in the first instance, as will just expel 

 the water of combination without inducing contraction. A temperature of 600 F. is 

 well adapted to effect this object, provided it be maintained for a sufficient period. 

 When this has been carefully done, the silicate of alumina remaining is easily enough 

 acted upon by sulphuric acid, either slightly diluted or of the ordinary commercial 

 strength. The best form of apparatus is a leaden boiler, divided into two parts by a 

 perforated septum or partition, also in lead ; though on a very large scale, brickwork 

 set in clay might be employed. Into one of the compartments the roasted clay or 

 shale should be put, and diluted sulphuric acid being added, the bottom of the other 

 compartment may be exposed to the action of a well-regulated fire, or, what is 

 better, heated by means of steam through the agency of a coil of leaden pipe. In 

 this way a circulation of the fluid takes place throughout the mass of shale ; and, as 

 the alumina dissolves, the dense fluid it produces, falling continually towards tho 

 bottom of the boiler, is replaced by dilute acid, which, becoming in its turn saturated, 

 falls like tho first ; and so on in succession, until either the whole of the alumina is 

 taken up, or the acid in great part neutralised. The solution of sulphate of alumina 

 thus obtained is sometimes evaporated to dryness, and sold under the name of ' concen- 

 trated alum ; ' but more generally it is boiled down until of the specific gravity of 

 about 1'35 ; then one or other of tho carbonates or sulphates of potash or ammonia, 

 or chloride of either base, or a mixture of these, is added to the boiling fluid, and 

 as soon as the solution is complete, the whole is run out into a cooler to crystallise. 

 Tho rough alum thus made is sometimes purified by a subsequent recrystallisation, 

 after which it is ' roched' for the market a process intended merely to give it the 

 ordinary commercial aspect, but of no real value in a chemical point of view. 



Alum Cake. This substance owes its value to the amount of sulphate of alumina 

 it contains, and is in fact another means of making soluble alumina accessible. Wo 

 have already seen the many attempts to obtain alumina from clay, and the tedious 

 nature of the operation of solution in acid, as well as the long after-processes of lixi- 

 viation and conversion into sulphate of alumina, or into alum, by rebelling or 

 crystallising. Mr. Pochin, of Manchester, has found a method of removing all the 

 difficulties, both of the first and after processes. He uses very fine china clay, free 

 from iron, heats it in a furnace, mixes it thoroughly with acid, and finds that, when 

 the process is managed carefully, the combination of tho alumina and sulphuric acid 

 is not only complete, but so violent that he is obliged to dilute his acid considerably, 

 in order to calm the action. When mixed, it is passed into cisterns with moveable 

 sides, where, in a few minutes, it heats violently and boils. The thick liquid 

 gradually becomes thicker, until it is converted into a solid porous mass the pores 

 being made by the bubbles of steam which rise in the mass, which is not fluid enough 

 to contract to its original volume. Tho porous mass is perfectly dry, although 

 retaining a large amount of combined water. It retains, of course, all the silica of 

 tho original clay, but this is in such fine division that every particle appears 

 homogeneous. The silica gives it a dryness to the touch not easily gained by the 

 sulphate only. 



When pure sulphate of alumina is wanted in solution, tho silica is allowed to pre- 

 cipitate before using it, but, in many cases, the fine silica is no hindrance ; then the 

 solution is made use of at once. 



Some quantity of alum has been exported from China, chiefly to India, within a 

 short period. The Chinese use alum very largely in their cements. 



The alum mines are in tho neighbourhood of Peh-kwan harbour, 2 9' 10" N., 

 12 32' 6" E. Ten alum-making establishments appear to exist there, and the pro- 

 cess, as described in the North China Mail, is similar to that employed where, in this 

 country, the alum-shale is used. 



AXiUMC EARTH. A variety of aluminous schist found associated with the 

 lignites of tho tertiary beds, especially on the Khine. 



AXiUM, FEATHER. A hydrated sulphate of protoxide and sesquioxide of iron, 

 called by Dana Halotrichite. Tho name ' feather alum ' is also sometimes applied to 

 a hydrated sulphate of alumina, known also as Alunogen. The latter is found 

 crystallised in a close mass of fine, white, flexible needles, of a feather or hair form, 

 and has been, like a few other substances, called hair-salt. It is also found with 



