ALUM. —Spence s Process. 171

The most important changes which the manufactureof alum has undergone in the last ten years arc thesubstitution of ordinary sulphuric acid for the sul-phuric acid generated by the prolonged exposure tothe air of iron pyrites, and the general substitutionof ammonia for potash. The direct treatment ofaluminiferous minerals with sulphuric acid waspatented in 1845 by P. Spence, of Manchester, andwas brought into practical use in 1847 by the firm ofSpence & Dickson. The importance of this stepmay be judged from the fact that while formerly GOtons of the Berkshire oolitic schist were required toproduce 1 ton of potash alum, the manufacture of 1ton of ammonia alum by the new process does nottake more than three-quarters of a ton of the samemineral. The schists used by Mr. Spence are foundunder the coal-beds of South Lancashire. Thisschist, which derives its black colour from the pre-sence of carbonaceous matter, is piled up in heapsof from 4 to 5 feet in. height, and calcined, as alreadydescribed, at a heat bordering upon redness. Theobject of this process is to make the alumina morereadily soluble in sulphuric acid. If the temperatureis too high, a partial vitrification (fritting) results,and the alumina is rendered almost insoluble in theacid. The calcination lasts ten days, a certainquantity of fresh schist being added daily. Whenthe operation has been well conducted, the residueis soft and porous, and of a light brick red. It isplaced in covered vessels, each capable of holding 20tons, and digested for tliirty-six to forty-eight hourswith sulphuric acid of spec. grav. 1 '35. The mixtureis kept at the heat of 230° Falir. (110° C.), by a firebeneath, and by driving in the vapour of an adjoiningboiler containing gas-liquor. This simultaneoustreatment gives excellent results, provided that theacid is always kept in excess. The ammoniaeal saltspass into the digesters, and are there decomposed bythe acid. The excess of ammonia is afterwards setat liberty by the addition of a certain quantify oflime. The ammoniaeal liquor flows into cisterns,and is stirred continually while cooling, in order todetermine the formation of small crystals. Thesecrystals are subsequently dried, and then washedwith the mother liquor that drops from the blocks ofalum. Not a trace of iron is found in these crystals,though the mother liquor contains it in abundancein the state of sulphate. This result is commonlyobtained by means of rapidly redissolving andrecrystallizing the alum. Spence arrives at the sameresult by means of steam, without addition of wateras a solvent. The crystals are thrown into a hopper,at the bottom of which they come in contact with acurrent of steam, which rapidly dissolves them. 'Fliccrystals and the steam are in such proportions thatall the crystals are dissolved when all the steam iscondensed: 4 tons of crystals are thus dissolved inthirty to forty-five minutes. The solution flows intoa leaden tank, where it stands for three hours, de-positing a certain quantity of matter insoluble bothin acid and water—probably basic sulphate of alumina.The liquor is thence run into cylinders, the bottomsof which are made of Berkshire flagstone, and which

are 2 yards in diameter, with movable sides. Afterstanding from five to eight days, the sides are re-moved, and a cylindrical mass of crystallized alum isfound. After standing again for eight days, a holein the bottom of the cylinder is opened, and a quan-tity of liquid escapes. The mass is generally 18inches thick at the bottom, and 1 foot in the sides,and contains 3 tons of marketable alum. The liquidwhich escapes contains another ton. The blocks ofalum obtained by this process often present a faintrose shade, which a hasty observer might attributeto metallic impurities. This tint is due to the pre-sence of organic matter, probably a trace of aniline,derived from the gas-liquor.

The substitution of ammonia for potash is notpeculiar to those establishments which make directuse of sulphuric acid. In consequence of the rise inthe price of potassic salts, ammonia has been intro-duced into the manufactories which still work onthe old method, not only in England and in France ,but in Germany . In 1851 the great Bonner Worksmade exclusively potash alum. Now, with the ex-ception of some 300 tons, its entire yearly productionis ammonia alum. Many samples sold as potashalum contain a proportion of ammonia.

Production of Alum from Rocks containingAlumina. —Alum has been produced from variousrocks of which alumina forms a constituent, by aprocess similar to that which naturally gives rise tothis body in volcanic districts. The operation isconfined to one or two isolated works on the Conti-nent , and those are conducted in juxtaposition withmetallurgic operations where sulphurous acid is inabundance. At Lintz, on the Rhine , where there isa spelter work and plenty of zincblende, zinc sul-phide, the sulphurous acid evolved during the roastingof the ore is conducted over the aluminous mineralmoistened with water. Basalt and copper slate arethe materials employed, and when these are exposedto the action of the gas and sufficient moisture andair, decomposition ensues, aluminium sulphate, withsulphates of iron and copper, are formed, which areseparated from the insoluble residue by steepingwith water, and the subsequent treatment for obtain-ing the alum is on the same principle as thosemethods which have been already described. Insuch operations there is always a quantity of sodiumalum produced, on account of the presence of thisalkali in the basaltic mineral. The sodium alum isformed from the commencement, and the amountcorresponds of course with that of the soda in themineral, which in many basalt rocks is from 5 to 6per cent.

Alum from Clay. — Ciiaptal and Alban, inFrance , devised a different process for the prepara-tion of alum from clay. Clay consists of basicaluminium silicate, free alumina, and by the action ofsulphuric acid aluminium sulphate is formed withseparation of silicic acid. Ripe-clay answers thepurpose best; but that the acid may more readilypenetrate the mass, the clay is submitted to a pre-vious calcination to increase its porosity, and morefully oxidize the iron which it may contain. Fresh