MANUFACTURES.

431

Manufac-

tures.

cious baskets made for the purpose. When well illumi-■ naterl, the crystalline surface of the roof, pillars, andsides of a large pit make a glittering and magnificentappearance, which seldom fails to have a very impres-sive effect on the mind of a stranger. Fresh air is con-veyed from the mouth of the pit by means of a tubewith a pair of forge bellows fixed to it; and thus a per-petual current is preserved between the outer and innerair. The pits, at the greatest depth, are dry, and of anagreeable temperature.

The largest rock salt-pit in Cheshire is one in thetown of Wilton. This has been excavated in a circularform to 108 yards in diameter ; its roof is supported by25 pillars, each three yards wide at the front, four at theback, and six yards in the sides. Each of these pillarscontain 294 yards of rock salt, and the whole area ofthe pit, which is 14 yards hollow, includes 9160 super-ficial yards, or little less than two acres of land.

The quantity of rock salt delivered annually from thepits in the neighbourhood of Northwich, is from 50,000to 60,000 tons. Hardly more than one-fourth of this isrefined in England, the remainder being exported to va-rious parts of the Continent. The salt is conveyed downthe Mersey in vessels of from 50 to 80 tons burthen toLiverpool , where it is reshipped for foreign Countries, orkept to be refined.

Besides the great quantity of salt obtained from therock, an immense weight is supplied by the brine pits,not less than 45,000 tons being annually manufactureda t this town, or in the vicinity. The usual depth of thesprings is from 20 to 40 yards ; and they are situated ona hill at some distance. The brine stream is raised bya steam engine, and conveyed through very long troughs

the brine pits. The process of extracting the salt isa ccomplished by heating the liquor in iron pans of from20 or 30 feet square, and about 14 inches deep. When itboils, a light scum rises to the top, which is taken off,a nd the liquor reduced to a lower degree of heat; thestream arising is made to evaporate as quickly aspossible'; and the salt collecting into crystals forms acrust on the surface, which, sinking afterwards to theb °ttom of the pans, is thence removed once or twice inevery twenty-four hours. The salt made in this districtI s distinguished into stoved or lump salt, common salt,ar ge-grained flaky salt, and fishery salt

.(615.) Lump or stoved salt. —In making this salt, therin e is brought to the boiling heat, which in brine fullysaturated is 226° Fahrenheit. This temperature is con-■nued during the whole process; and as the evaporationProceeds, small flaky crystals continue to form them-of 'f S ’ at '^ to i a 'l to the bottom of the boiler. At the endWat r ° m e '“bt to twelve hours, the greatest part of theonl so lntion is found to be evaporated ; so muchof the 6 *' 1 "' aS barely to cover the salt and the bottombasket ^ an ^ le salt is then removed into conical wickerdried j S termet ^ barrows, and, after being well drained, isMoves, where it sustains a loss of about one-

On h ° f ' tS Weight

of car^ 16 aPPboat'on of heat to the brine, a quantityiron h b me > a nd sometimes a little oxide of

cess'of 0 r "[bich had been held in solution by an ex-moved ? ar , ?' c ac 'd, are separated, and are either re-bottom of !, lmmin - or are allowed to subside to thewith some ? ? an alon S with the salt first formed andout. Thes^t ate bme, and are afterwards rakedSome brinp« W ° °P era b° ns are called clearing the pan.

scarcely require them at all, and others

only occasionally. The whole of the impurities, how- Mechanicalever, are not thus removed; for a part subsiding to the Processe8,bottom, forms a solid incrustation, termed by the work- * _v_1men pan-scale. The portion of this which is lowestacquires so much induration and adhesion to the pan,that it is necessary to remove it once every three orfour weeks by heavy blows 1 with a pick-axe. Thesesediments are formed also in making the other varietiesof salt.

Common salt .—The brine for forming this salt is raisedto a boiling heat, with the double view of bringing it asquickly as possible to the point of saturation, and ofclearing it from its earthy contents. The fires are thenslackened, and the evaporation is carried on for twenty-four hours with the brine heated to 160° or 170° Fahren-heit. The salt thus formed is in quadrangular pyramidsor hoppers, which are close and hard in their texture.

The remainder of the process is similar to that of mak-ing stoved salt, except that, after being drained, it iscarried immediately to the store-house, and not after-wards'exposed to heat, an operation confined to thestoved salt.

Large-grained /a% salt is made with an evaporationconducted at the heat of 130 or 140 degrees. The saltthus formed is somewhat harder than common salt, andapproaches more nearly to the cubic shape of the crystalsof muriate of soda.

Large-grained or fishery salt is prepared from brineheated only to 100° or 110° Fahrenheit. No per-ceptible agitation, therefore, is produced in the brine,and the slowness of the process, which lasts from sevenor eight to ten days, allows the muriate of soda to formin large and nearly cubical crystals.

For ordinary domestic uses, stoved salt is perfectlysufficient. Common salt is adapted to the striking andsalting of provisions, which are not intended for seavoyages or warm climates. For the latter purposes,the large grained or fishery salt is peculiarly fitted.

All the pans were formerly, and are still in manyworks, heated by furnaces beneath them with fluesdrawing all ‘ round, and the fuel is commonly coal;but recently the evaporation is produced by means ofsteam, as described below. In the former case, each ofthe pans, which are set securely upon masonry with thefurnace below, stands in a covered building, with a pyra-midal roof formed of boards sloping downwards, butwith a considerable interval between each to keep offthe rain, but allowing, at the same time, a free passagefor the steam of the boiling brine to pass off.

(616.) Evaporating by steam .—Two patents have Patent ap-been granted for evaporating by means of steam, instead * ot

of the usual furnace and flues, in one of which a vessel ^ oracontaining the brine is inserted in a steam boiler, or in-timately connected with it, the brine vessel being openon its upper part.

In the second by Mr. Furnival, the brine is, as inthe other, first pumped into an open vessel at A, fig. 5, PI. xlvi.plate xlvi., inserted in or fixed upon a close steam 5-boiler B; but the boiler, instead of containing freshwater, contains a still stronger brine than that in theupper vessel. This is brine which has been operatedupon in the upper boiler, and let down into the lowerone to be finished or evaporated to such a degree thatnothing but salt remains. To effect this, adivisionof sheetiron C is made in the top or upper side of the lowerboiler in a diagonal direction, containing an opening d,similar to a dormer window in a leaning roof. This3k 2