SODA.— Black-ash Waste. 823

reasons why the temperature during the lixiviationmust be kept as low as is consistent with a thoroughexhaustion of the balls. But it is well known topractical men that a high temperature does muchmore harm in the weak tank than in the strong one;that is to say, it causes much more sodium sulphideand caustic to be formed where water acts uponnearly exhausted balls, than where a strong lye actsupon fresh balls. On the other hand, unless there is agood heat (140° to 150°) in the strong tank the ballsnever dissolve as well as they might, and are not sowell exhausted. This aim can be attained with morecertainty by substituting steam for warm water inthe lixiviating tanks, and in the case of revolver-balls this is altogether indispensable. In this case,too, steam-pipes in each tank just dip below theliquids; thus they are not in the way in cleaningout, and no currents are caused by the action of thesteam. The water is run to the weak tank almostor quite cold; it is only slightly heated (to about110° Fahr.) in the intermediate tanks; but in thestrong tanks it is kept at 150°, or even more, withoutdoing any harm.

Another important precaution to observe is this,that the balls, when once they have become wetted,must always be kept submerged below the surfaceof the liquid, and if they stand out, must be carefullypulled under with a hook. Unless this is done themoist calcium sulphide is speedily oxidised to a greatextent, and the resulting sulphate, sulphide, &c.,destroy an equivalent quantity of sodium carbonateby double decomposition.

Tank Waste .—This in most factories is a realwaste product, and is besides a source of nuisanceand expense. It ought to be of fine grain, fromthe size of a mustard seed up to that of a pea ; anylarger lumps or clots are almost sure to contain toolarge a proportion of undissolved sodium salts.From the appearance of the waste it is pretty safeto judge upon its being properly lixiviated; but it isalways advisable to test a sample from each vat.This can be done thus:—50 grams, of the well-mixed sample are shaken up for some time withtepid water, and diluted to 1000 c.c. Of the clearsolution, 100 c.c. (= 5 grams, waste) are evaporatedto dryness, with addition of a little ammonium car-bonate. This is done in order to decompose anycalcium bisulphide or hyposulphite, which wouldinterfere with the alkalimetrical test. The dryresidue is heated to expel the ammoniacal salts,dissolved in water, and the filtered solution testedfor soda with standard sulphuric acid. The pro-portion of soluble soda thus found ought not toexceed one-tenth of a per cent. It is also interest-ing now and then to test for the total soda, in-cluding the insoluble portion, which is of courseirrecoverable from the waste, but whose proportion,if found excessive, affords some guidance for thecarrying on of the balling process. The total sodacan be ascertained thus :—18-00 grams, of the wasteare heated in a porcelain or even iron dish withsulphuric acid, of 100° Tw., till it is thoroughlydecomposed and converted into a paste. The

excess of sulphuric acid is driven off by ignition,boiling water is added, and the contents of the dishscraped out with a wooden spatula, and transferredto a glass cylinder of 250 c.c. capacity. Here alittle lime cream is added, which neutralizes anyfree acid remaining, and precipitates all magnesia.The cylinder is filled up to the mark, and allowed tostand till the liquid settles; 50 c.c. of the clear liquidare taken out, 10 c.c. of a concentrated solution ofbarium hydrate are added, filtered through a dryfilter; 50 c.c. of the filtrate taken away, and a currentof carbonic acid passed through. The liquid is thenboiled, filtered, and the clear filtrate tested withstandard acid. Each c.c. standard acid will corre-spond to 1 per cent, of soda in the waste, from allsodium compounds, soluble and insoluble. If noallowance for the bulk of the waste itself were made,18-60 grams, would have to be employed; 18-00provides for the above.

In most factories it is usual to remove thetank waste as soon as possible, as after a short time—sometimes after one day’s keeping, sometimes alittle later—a decomposition sets in by the aid ofthe moisture present in large quantities, and of theatmospheric oxygen, which causes the formationof soluble calcium hydrosulphide, bisulphide, andhyposulphite—not to mention insoluble and in-nocuous products of the action. The carbonic acidof the air acting upon these compounds, moreespecially the two former, causes the evolution ofhydrogen sulphide, which of course amounts to agreat nuisance. A sign of the oxidation is this—that the mass gets heated; after a few days ofexposure in the loose state, it may even becomered-hot. The more it heats, the more it alsogenerally smells. This heating and its consequences—viz., the formation of soluble calcium sulphides—canbe avoided to a very great extent if the tank waste isnot allowed to lie in loose heaps, or mixed withother refuse (ashes, &c.), which keeps it in a porousstate and accessible to air. If it is laid down inan unmixed form, and beaten down with shovels soas to form a level, dense surface, this is soon madealmost impervious to air by the formation of calciumsulphite and sulphate ; and when treated in thismanner the tank waste is in itself not in the leastoffensive—no sulphuretted hydrogen is given offfrom it, and in most chemical works many of thefloors of the working sheds are formed in this way.The tank-waste floors do not, however, stand anyother but foot traffic, as they are cut up very easilyby cart or even by barrow wheels; they can onlybe used for barrow traffic by bedding in iron orwooden runners for the barrow-wheels. The samekind of floor answers very well for the drying platesin brick-works. These applications dispose, ofcourse, of only a very small quantity of the tankwaste; and the further proposal which has beenmade—viz., to fill up the space between the flooringjoists of dwelling houses with it, in order to pro-tect them from dry rot—does not seem to have hadany practical success. Formerly the bulk of thetank waste was disposed of by putting it in heaps