ALUM.—Washing.

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Having prepared the aluminous liquor, and ascer-tained the amount of salts contained per gallon, thecrystalline flour is precipitated by gradually addingto the aluminous liquor the concentrated solution ofthe precipitant, the mixture being kept briskly agitatedtill the whole of the powder is thrown down. Aslanting plane is fixed adjacent to the precipitatingvessels, on which the workmen shovel the subsidedpowder, for the purpose of freeing it from the impuremother liquor of the vessel; the portion of lieadhering to the crystals drains off, and runs back tothe mother liquor; still it does not wholly separateby this means, a small portion being mechanicallyretained by the mass of powder, which produces thebrownish-yellow colour of the crystals after the firstdraining. To remove the colour recourse is had toanother process called “washing.” For this, it issufficient to stir the powder with a small quantity ofwater in an appropriate tank or vat. The impurity

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mixed with the crystals is taken up by the water, aslikewise a small portion of the alum, and when thesubstance has subsided the liquor is drawn off bymeans of a siphon ; two washings are found sufficientto separate all adhering foreign soluble bodies. Thewater employed in washing the flour should be of aslow a temperature as possible, as the quantity of alumdissolved out is in proportion to the temperature ofthe water.

Fig. 5 is a view of the precipitating and crystalliz-ing cisterns at one of the Ilurlet works; they areconstructed of stone, well adjusted and embeddedin tempered clay, on a site contiguous to the evapor-ating furnaces. Having ascertained the quantity ofsulphate of alumina per gallon in the concentratedliquor in those boilers, their whole bulk of lie is foundby gauging, at the temperature of G0° Fahr. (15°-5 C.).A corresponding amount of potassium chloride, orammonium sulphate, is then weighed out and placed5.

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in one of the cisterns, after which the plug is removedfrom the boiler, and the liquor conducted alongstoneware gutters to the coolers till it flows uponthe heap of chloride. Sometimesthe latter is graduallyadded to the liquor as it flows along in the gutters.Towards the end the mixture is well agitated toinsure a complete solution of the alkaline salt and auniform state of the liquor. In four or five days,when the plug is drawn out from the cistern and themother liquor run off, the bottom and sides of thecooler arc found lined with crystals of alum, generallysome inches in thickness. This is called first alum,and is removed from the vessel to the draining stage,which is a raised platform in a sloping position, sothat the mother liquor drains off from the heap, asshown in Fig . G.

The first alum, when drained, is washed to removeas much of the mother liquor from it as possible.For this purpose tubs are filled with second alummothers, or liquor which has been used in washing

in a subsequent part of the process; and the alum,placed in wooden sieves, is rinsed in this liquor, andthrown on the stage.

The second alum boiler, which is a deep stonecistern covered with wood, Fig. 7, is then preparedby running in a quantity of liquor which had beenpreviously used in washing the second alum ; steamis next introduced, till the liquor has nearly attainedits boiling point, when the washed and drainedcrystals arc thrown in, and constant agitation keptup. This is continued till as much alum has beenadded as will increase the specific gravity to 00°Twaddle, and the temperature is raised as much aspossible by the steam, which is then shut off, and theboiler being closely covered up, the liquor is allowedto remain at rest for twelve hours, for all the impuritiesto settle; the plug is then removed, and the clearliquor runs off to the second alum coolers, which,like the preceding; are sunk stone cisterns. In theseit remains for four or five days, when the sides and