SODA. —Manufacture of Sulphate of.

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at 15° C.; at 100° C. water dissolves its own weightof the salt. It is insoluble in alcohol. A crystallizedsulphate of soda is also known, containing 8 equiva-lents of water, and having the formula Na 2 S0 4 8II 2 0.

Sulphate of soda is a mild but efficient coolinglaxative or purgative salt. It was formerly the mostfavourite saline cathartic in this and other Europeancountries, but since the beginning of the presentcentury has gradually been displaced by the sulphateof magnesia.

Manufacture of Commercial Sulphate of Soda( Salt-cake ).—By far the largest quantity of thisarticle is manufactured for the purpose of servingas a material in the manufacture of soda ash; andthis is nearly always done in the alkali works them-selves, one of whose principal branches it forms.Only exceptionally sulphate of soda is made as aby-product from the manufacture of muriatic acid,the relation between them in alkali works beingalways the other way; in the former instance it ismade in cylindrical cast-iron retorts, very similarto those used for nitric-acid making, and the acid iscondensed in a series of stoneware Woolfe’s bottles.This product is known as “ cylinder cake,” andfrequently contains a good deal of undecomposedsalt. On the other hand, in some eases, but rarelyin this country, sulphate of soda is made purposelyfor glass making; and as in this case (except forcommon glass) it must be as free from iron aspossible, it is made in leaden pans. The great bulkof sulphate of soda is made in the large cast-ironpans or pots first introduced by Gamble or Lee(it is not quite certain by which of these in the firstinstance), or by the recently introduced process ofHargreaves and Robinson, which will be afterwardsdescribed ; but it is very probable that the mechani-cal furnace introduced by William Jones, ofMiddlesborough, which will be described below, willcause an important revolution in this manufacture.

The raw materials for sulphate of soda makingare sulphuric acid and common salt (sodium chloride).Both of these are treated of in their respectiveplaces; and it need only be mentioned here, thatthe sulphuric acid need not be pure, nor any moreconcentrated than as it is furnished by Glover’stowers, or even by the first acid chamber ; it is onlywhen sulphate nearly free from iron (for glass-making) is required, that attention has to be paid tothis point in the acid. The common salt used in thiscountry is always that from Cheshire, which is notonly cheap, but also remarkably pure. The saltused abroad often contains very many impurities(especially calcium sulphate), which greatly injurethe quality of the salt-cake, and subsequently thatof the soda ash made from it.

These two materials, in acting upon each other,ultimately produce the following result:—

2NaCl + I1 2 S0 4 Na 2 S0 4 + 2HC1.

This decomposition does not take place quitesmoothly and all at once, but gradually, since thesulphuric acid always inclines to form acid salts(hydrosulphates). For this reason the hydrochloric

acid is only partially driven off at a moderate tem-perature, and the remainder requires a very muchhigher temperature for its elimination. The firststage (at a moderate heat) takes place thus—

2NaCl + n a S0 4 = NaHS0 4 + IIC1 + NaCi.

The second stage (at a stronger heat)—

NaHS0 4 + NaCl = Na 2 S0 4 + I-ICl.

The second stage, however, always partly sets induring the first period. Anyway there are always(except in Jones’ machine) two stages in the manu-facturing operation as well—viz., first, moderateheating in a metal vessel (pan); and secondly, strongheating (ignition) in a furnace. The hydrochloricacid evolved in the first stage, from the pans, isboth more concentrated and pure than that fromthe second stage, the furnacing, where a little sul-phuric acid always accompanies the escaping gases.

The usual process of salt-cake making in decom-posing pans is always carried out in these twosuccessive stages—viz., first, in the pan itself, wherethe decomposition is not carried right through; andsecondly, in the furnace (“roaster” or “drier”),where it is completed. The process in the pansthemselves is essentially the same everywhere,although there are not unimportant modificationsin detail in the shape and in the mode of setting andfiring the pans. But a much larger difference existsin the furnaces, which may be either “ closeroasters’’ — that is, muffle furnaces, where theflame does not come into direct contact with thebatch; or “open furnaces”—that is, reverberatoryfurnaces, where the flame plays direct upon thebatch. The former construction is more usual inLancashire, Scotland , &c.; the latter is the onlyone found on the Tyne. Each kind of furnace hasits advantages and its disadvantages, as will be seenhereafter. A third kind, where the flame first travelsunderneath the bed, and then directly over thecharge, has been tried with gaseous fuel, but doesnot seem to have met with much approval.

Decomposing Pan .—This vessel is cast of par-ticularly tough metal (the mixture of different brandsmost suitable for this purpose is considered a tradesecret by the comparatively few iron-founders whocast these pans), forming a segment of a globe, asseen from the diagrams. They are usually from 2to 3 inches thick near the top and from 5 to 7 inchesthick in the centre ; but a few manufacturers preferto have them made thickest about half-way up.The weight of a pan, of about 9-| feet diameter and2 feet depth, is about 6 tons. In the majority ofcases their upper margin is either plain or providedwith a horizontal flange, and in this case they arecovered over by an arch, which leaves the sides of thepan quite clear, and allows it to be withdrawn andreplaced without disturbing the brickwork. This is acertain advantage, though more in time than in money;but for all this the construction shown in Plate I. ispreferable, where the pan is provided with an up-standing rim in "addition to the flange, and thecovering arch is sprung from this rim itself in the