ALUM.— Produce, Impurities, and Chemistry.

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annexed figure, is employed. Fig. 15 shows themanner of effecting the solution of the aluminiumsulphate. B 1 B 2 is 3 , &c., are a series of pits, furnishedwith pipes, A 1 A 3 A 3 , &c., for the purpose of convey-ing mother liquor from one to another, the pipesbeing stopped with taps or cocks, as seen in thefigure. Water is run from the service tank, D,through the pipe, C, into the pit, B°, till it flows overthrough the pipe, A 6 , and passes through the pipe,E E, till it enters B 1 , through which it rises till thepit is filled, exhausting the contents of the solubleingredients in its ascent. When the liquor has stoodfor some days, a fresh supply of water is continuedas before in B 6 , by which the liquor in B 1 is forcedover through A 1 , the plug being removed into thepit, B 2 , containing fresh calcined shale ; the liquorascends through the shale in this pit as in b 1 , andwhen it has remained two or three days in contactwith it, a fresh supply of water is allowed to flowinto B r> , which forces the liquid contents of B 2 intothe third tank, and the treatment repeated as long asthe workmen find it necessary to bring the liquorto 28° Twaddle, or such higher strength as is required;and when the proper degree of saturation is attained,the solution is drawn off by the stopcock, r. Assoon as the shale in any of the pits is found to be

exhausted, it is removed, and a fresh supply intro-duced, and the lixiviation proceeded with as before.

Of course, as the operation goes on week after■week, it will become necessary to commence thesupply of water from the service tank, D, to thedifferent pits in succession, which may be done asabove described, or by allowing it to run at oncedown the pipes, A A A, from the respective taps inthe pipe, C. By these arrangements the labour ofpumping the liquor from one pit to the other will besaved, the process generally rendered more con-venient, and the result will be a more perfectexhaustion of the shale,

Produce. —The great variation in the characterand richness of the ores does not admit of even aproximate relation being established between thequantities employed and the amounts of alum pro-duced in different works. At Valmunster, from17,300,000 lbs. weight of alum shale, 400,000 lbs.of alum, or 2-3 per cent., are obtained, for the pro-duction of which 100,000 lbs. of sulphate of potassaare required, and 1,400,000 lbs. of coal consumed.

The slate at Liege yields 2 per. cent of alum, andthat from other localities only from 1 to 4 per cent.

Impurities. —Alum always contains a little iron;that from Liege contains 002 per cent.; that from

Fig. 15.

Gavelle, near Paris , 0'08per cent.; that from Ayeyron,0T1 per cent.; and English alum about 0T2 per cent.This amount of iron is sufficient to impair the delicatecolours used in calico-printing; it can, however, beremoved by recrystallization, or as prussian blue, bymeans of potassium ferrocyanide (K 4 FeCy 6 ). TheRoman alums—a name given to all varieties of alumimported from Italy —are sufficiently pure for inline-:diate use, and are highly prized on account of theminuteness of the quantity of iron which they contain.

Roman alum commonly consists of small discon-nected crystals, covered externally with a kind ofreddish mud; on the alum being dissolved in waternone of this coating is acted upon, and the quantityof iron alum present in the solution does not exceed(K)05 per cent. Alum of this description produces ablue precipitate with potassium ferrocyanide onlyafter standing for several hours, while ordinary alumaffords a precipitate in a few minutes. Roman alumis generally crystallized in the form of octahedrons,though cubical crystals are occasionally found. Itsreactions differ from those of ordinary alum in manyrespects.

Chemistry of Alum. —Potassium alum requiresabout 18 parts by weight of cold, and less than 1part of boiling water to dissolve it, and crystallizes

very readily in large regular octahedrons, of whichthe apices are always more or less cut off. Its tasteis sweetish, and very astringent. It is an acid salt,reddens vegetable blues, and dissolves metals withevolution of hydrogen; in the air it changes veryslightly. Its water of crystallization amounts to 45'5per cent, of its weight, or 12 atoms; thus it loses itswater of crystallization with great facility at 197°-6Fahr. (92° C.). The fused salt in losing this waterbecomes viscid and intumesces, leaving calcined orburnt alum, which is sometimes used as a causticwhich dissolves in water, although but slowly.

The octahedral crystals of potassium alum consistof sulphuric acid, aluminium, potassium, and water,and have the formula A1K(S0 4 ) 2 12II 2 0.

When octahedral alum is digested with hydratedalumina, and the solution allowed to evaporate spon-taneously by exposure, cubical crystals arc formedwith a simultaneous separation of a basic salt ofalumina; the crystals have an acid reaction, and thesame properties as ordinary octahedral alum.

Loewel prepared pure potassium and ammoniumalums, crystallized under the ordinary octahedralsystem, and obtained cubic crystals of potassium alumby dissolving the octahedral crystals in three or fourtimes their weight of water, heated from 72° to 81°