SODA.— Black-ash Liquor. Salting-doivn. 827

that which was first made and has been paddledthere, might be expected to be less caustic than thatfrom the front end, which is made later on, and it isso mostly, although not regularly.

The second table shows that all the sulphur iscarried through the process into the ash, and noneof it volatilized, as has been stated by Muspratt.On the contrary, its proportion is found to increase,undoubtedly partly from mechanically carried oversodium sulphate, and partly from the action of thesulphurous acid contained in the furnace gas.

A second system of surface evaporation is notpractised very much in this country, but more so inFrance . It is, at the same time, the oldest systemof all, and consists in combining evaporation andcalcination in the same furnace. The bed of the fur-nace is shaped like a disk, and walled with very tightjoints; it isbroughtto a bright red heat before runningin any liquor, so that a crust is formed at once whichprevents the liquor from soaking too much into thebrickwork; the evaporation is then continued withfurther quantities of liquor, and when a sufficientquantity of salt is collected, it is at once furnacedoff and made into finished ash. This process givesa good quality of ash, but it is not adapted for thelarge-scale production of our times.

The process of bottom evaporation is also carriedout in two different ways, viz., in flat pans and inboat pans. The former, which are the commonones in Germany and France , are built to someextent like salt pans, with hoods to carry away thesteam, and provided with a depression along theside opposite to the fire. A common dimension is25 feet long by 8 feet w-ide, by 20 inches deep. Thebottom must be perfectly level and the rivets mustbe counter-sunk, as it is necessary to rake the saltcontinually away from it into the depression justmentioned; otherwise it would burn fast to thebottom, and the latter would be burnt away in anextremely short time, apart from the waste of heatacting through the iron and salt together. Thesepans have generally separate fires, and the smoke isburnt as much as possible; frequently the flameescaping from them serves for a preliminary heatingof other pans. Each pan is filled up twice everytwenty-four hours, and the salt accumulating in thedepression is taken out, with a long, bent, perforatedshovel, four times during that time, in order to bedrained in a kind of hopper, from which the mothersrun back into the pan. The latter thus accumulatemore and more, and after eight or ten days (onlywith very good liquors after twenty-one to twenty-four days) the operation must be interrupted, becausethe fished soda ash becomes too weak. The remain-ing liquid is then evaporated till it shows (hot) T41to 1'44 spec. grav.; the salt then separated on cool-ing mostly consists of sodium sulphate, and is workedup again in the ball-furnaces. This plan seems toinvolve much labour, expenditure for fuel, and wearand tear of the pans, and is not at all favoured byBritish alkali-makers.

The only system of bottom evaporation in exten-sive use in this country is that in the “ boat pans,”

first introduced by Gamble. These are sometimesmade of cast iron, 12 feet -long, 8 feet wide, and 3feet deep ; sometimes of wrought iron, from 20 to30 feet long, from 7 to 10 feet wide, and from 2£to 3 feet deep in the centre. Their shape (shown inthe wood-cuts given previously) resembles to someextent that of a boat, and they are set in such amanner that the centre or deepest part of the bottomis not at all touched by the flames, as it either restson an arch or on a solid pillar, as explained abovein describing the ball-furnace. The fire only touchesthe slanting part of the sides, not quite up to thepoint where they curve upwards, so that there re-mains enough room for the brickwork to supportthe pan, and at the same time enough room insidefor a margin during the boiling down, as the surfaceof the liquor should of course never be lower thanthe highest part of the fire flues. In these pans thesalt separating during the evaporation naturally andwithout any labour collects in the central depression,which is not touched by the fire, and is thus protectedboth against burning of the plates and against adisturbance of the settling by the ebullition of theliquid—the centre being just the quietest part ofthe pan. It is also very easy to “ fish ” out the saltfrom this central depression; and this kind of pansis indeed quite specially adapted for the Lancashire process, in which it is purposely avoided to carbonatethe liquor even partially by means of the furnacegases, as it is the intention to separate the productsinto fished ash (of 55 to 57 per cent.), on the onehand, and “ caustic ash ” or caustic soda, on theother hand. “ Caustic ” ash is the product obtainedby boiling down the mother liquor from the fishedash to dryness; the preparation of caustic soda willbe described separately. The “ caustic ash ” sus-tains from 5 to 8 per cent, of sodium hydrate; andalthough its colour is very inferior, it is in gooddemand by paper-makers, soap-makers, and others,to whom its causticity is quite welcome. Its alkali-metrical test is 46 to 50 per cent.

Another kind of evaporating pan was pro-posed by Gossage in 1853. It is a double pan,consisting of two compartments connected by alarge tube, one of which only is heated, whilst theother one serves for receiving the salts. The liquorsare pumped continually from the cold pan into thehot one, and consequently overflow from the latterback into the former, where the salt crystallizes out,and the mothers are pumped back again. Thissystem seems to be arranged in a most rationalmanner; but there must Lave been practical diffi-culties in the way, as it has not made its way amongalkali-makers.

A modification, which refers less to the construc-tion than to the working of the pans, has beenintroduced by Kuiilmann. It consists in fishingout the salts during evaporation of a fixed quantityof liquor, until the volume of the latter has beenreduced to a certain point. By varying the relativevolumes, salts of different degrees of strength canbe obtained by fishing. Thus, when ordinary tankliquor of T286 specific gravity is boiled down to