The vessels employed in the CRYSTALLISATION are pans or tubs, usually circular and about 7 or 8 feet wide, by 2-1/2 to 3 feet deep; and are generally set on the ground, or are embedded either partially or wholly in it. The saline liquor being pumped or run into them at a little below the boiling temperature, crystallises as it cools; the only interference being occasional stirring or agitation, to prevent the formation of large crystals, which would be inconvenient in the subsequent part of the process. The time occupied in the crystallisation varies, according to the size of the ‘crystallisers,’ and the weather, from 3 or 4 to 8 or even 10 days. The ‘mother-liquor’ of the ‘crystallisers’ is pumped back into the evaporating pans for further concentration. The crude blackish salt (hydrochlorate) thus obtained is contaminated with tarry and oleaginous matter, free acid, water, &c.; from part of which it is freed by exposing it in a layer about 4 inches deep, on a cast-iron plate gently heated by a zigzag flue of a small furnace, until all the water is expelled; care being taken that the heat never rises high enough to volatilise the salt. This operation is generally performed under a dome, or the expanded throat of a large chimney. The salt will now have become of a greyish-white colour, and is ready for the next operation. [Illustration] The crude dried salt of the last process is finally purified by sublimation. For this purpose cast-iron-pots lined with clay, and heated from below and by flues round their sides, are employed. (See _engr._) The crude grey salt is beaten down into these pots until they are about 2-3rds filled, when the heads or capitols are fitted on, and heat applied. The latter are very heavy, being usually made of lead (sometimes of iron), and have the form of a dome, or a hemispherical cup, with a small tube or hole at the apex, in which a plug is loosely placed, to permit the escape of steam. These domes or heads are so made as to fit closely and firmly on the flat rim or flange of the ‘sublimers,’ and are retained in their places, during use, both by their weight, and by 2 or 3 clamps provided for the purpose. They are also furnished with 3 rings, set at equal distances, to allow of their being lifted off, or moved, by means of a pulley and chains. The due application and regulation of the heat is here of the utmost importance. If the temperature employed be too high, the sublimed salt will be contaminated with empyreumatic matter, while some of it will be carried beyond the dome and lost; and if it be extreme, the head may be altogether blown off, and the contents of the pan scattered about the building; whilst on the other hand, if the heat employed be too low, the resulting cake of sal ammonia will be soft, spongy, and either grey or yellowish. The proper temperature is said to be known by two or three drops of water readily boiling, and being dissipated in vapour, when