ature has risen to 400° Fahr., very little steam seemsto be evolved, although the contents of the pot stillhold 20 per cent, water. At about 460° Fahr. thecaustic contains as nearly as possible 60 per cent,alkali; at 470° Fahr., about 61 per cent.; and at 500°Fahr., about 64 per cent. About this time there isvery little motion in the pot, and its temperature soonrises above the range of a mercurial thermometer.
At this stage some extremely pungent matterhaving a strong ammoniacal odour is thrown off,owing to the action of the caustic soda carried awayby the steam, or perhaps by its action on thecyanides present.
The cyanides are destroyed with the formationof graphite (first observed by Dr. Pauli), whichcovers the surface of the pot.
Any boiling to be now observable is confined toa slight effervescent motion round the edges of thepot, and it soon ceases altogether. The pot isnow covered with a sheet-iron lid provided withan opening at one side, and the fire is urged asstrongly as possible.
The time has now arrived for the completeoxidation of the sulphur compounds. This canbe done either by means of nitre, or by blowingin air. In the former case, the nitre is introduced(a hand scoopful at a time) through the openingin the lid. Its action is at first very energetic,often causing a deflagration. As the contents ofthe pot approach “clearness,” the addition of nitremust be made with great caution, in order not tooverdo it; every now and then a small sample istaken out, solidified on a clean shovel, and triedwith a few drops of a solution of lead acetate. Whenthis causes only a very faint darkening by transmittedlight, the further addition of nitre is discontinued.From 40 to 45 lbs. of nitre suffice for 1 ton caustic.in the case of good liquors.
The process of oxidation by means of a currentof air was invented by Ralston, in 1860 andindependently in 1865 by Peter Hart ; it wasagain, without any essential modifications, patentedin 1870 by IIelbig, to whom in some Germanpublications all the credit is given. The currentof air is generated by an ordinary blowing enginewith short stroke for low pressure. There mustbe a receiver with a safety valve for any watercarried away from the blowing cylinders, if this, asusual, is lubricated by water, as in the pot it wouldproduce an explosion. The engine usually adopted inLancashire is double-acting, and if only intended forblowing two or three pots at a time, it has a 12-inchair cylinder and 12-inch stroke. The main deliverypipe has 2^ inches, and is provided with a reducingtee for each pot and with a waste-cock. When apot is ready for blowing, a strong 1J inch wrought-iron pipe, terminating at its lower end in a perforatedring of about 18 inches in diameter, or a tee of thesame length, is plunged into the pot, till the ring ortee rests on the bottom ; the upper end is joined byan india-rubber pipe and coupling screw to one ofthe branches of the air pipe. On starting the enginethe waste-cock must be left open for a few seconds,VOL. II.
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to remove any water. The fused mass is soon inviolent commotion, and after two or three hoursbecomes red hot. If the liquor has been good andthe boat pans nitred, three to four hours blowingsuffices; if no nitre has been employed in the pans,about eight hours. For inferior liquor much more(even thirty-six hours’) blowing may be necessary.It must not be overdone any more than nitreing; butthe caustic should become green by the formation ofsodium manganate, and a trace of sulphide is there-fore purposely left in. If the blowing (or nitreing) hasbeen overdone, it is remedied by throwing in a smallpiece of sulphur or sodium hyposulphite. From 25,000to 30,000 cubic feet of air are sufficient per ton ofcaustic. Nitreing occupies five or six hours, andabout 10 per cent, less pot room is necessary for itthan for the blowing, which causes a much moreviolent commotion. The reaction in this stageseems to be:—
5Na 2 S + 8NaN0 3 = 5Na 2 S0 4 + 4Na 2 0 + 8N.
After the oxidation a sample is withdrawn (whichis always coloured brown or even red) and testedfor its amount of total alkali. If intended for 60per cent, caustic, the contents of the pot must nowbe reduced with common salt. It may turn out 68to 70 per cent.; or if for 70 per cent, caustic, to 7072 per cent. The addition of the salt must be madevery cautiously, as it decrepitates on throwing itin, and causes an extremely irritating spray, andcools the pots, which must now be brought upto beat again, and allowed to stand for eight totwelve hours. The ferric oxide settles down tothe bottom, along with sodium aluminate, and thefine quality of the caustic entirely depends uponthe completeness of the settling; sometimes potsrefuse to settle at all, especially when over-oxidised.The cause of the ferric oxide only settling downafter the mass has been brought to igneous fusionis probably this, that it becomes much denser andheavier by ignition, as known to every tyro inanalysis. The clear settled caustic is now baled outinto the well-known circular sheet-iron drums,holding about 6 cwts. each, which are placed in twoor more rows round the pot, and filled by a mov-able iron shoot. If quite perfect, the caustic oughtto run down “ as clear as gin.” On approachingthe lower part of the pot, the workman proceedsvery cautiously ; and as soon as the caustic becomesdoubtful in quality, it is baled over into the next“ strong ” pot, till the workman comes to the“ bottoms.” These are either baled into separatedrums, and sold as bottoms, or else into iron bogies,in order to be broken up, dissolved by help ofsteam to 48° Tw., thoroughly settled, and the clearliquor causticized. They amount to 9 to 11 percent, in the caustic produced, and are generally soldas 54 per cent, strong; if stronger, they are reducedto this point by fished salts. The following areanalyses of bottoms (by Davies) —A, from a potvery red before salting; B, from a pot whose liquorshad been well oxidised in causticizing; C, fromliquors only partially oxidised in causticizing:—•
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