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mixed with the soap by agitation. The mixture isthen boiled for some hours; a small quantity of lyebeing added from time to time, if necessary, to pre-serve an excess of alkali until the soap is completelyformed, when the lye may be withdrawn and one ortwo waters, or else weak lyes, applied successivelywith agitation for the purpose of washing and puri-fying the soap. The resinous scum being removedfor another operation, the soap is conveyed to theiron frames to solidify. Yellow soap usually con-tains, to 1 part of soda, from 10 to 11 parts of oilyacid and rosin, with from 24 to 30 per cent, of water.This communicates to the soap a viscid consistencewhich is not altered by long exposure to air. Ac-cording to Richardson and Watts, the best plan ofpreparing the soap is to saponify the rosin and tallowseparately, and to mix the two soaps in the boiler,where they are retained in a state of ebullition forsome time, until a uniform mixture has been effected;salt is then to be added and the soap brought to themoulds. It is not advisable to mix the separatesoaps in the moulds, though this plan is sometimesadopted. The soap is finished when a cooled sampleis of the right consistence, and does not leave afilm. If it contains a large proportion of water, ithas just the same appearance as when it con-tains only a small proportion. When anythinglike 50 per cent, of water is present, it has beenadded while the soap was in the frames for thepurpose of fraud.
If tallow or other grease be employed in the manu-facture of yellow soap, instead of palm oil, the follow-ing is the mode of procedure:—2000 lbs. of the oil orfat, 600 lbs. of rosin, and from 150 to 175 gallons ofsoda lye of specific gravity T075 to 1T50 = 15°to 30° Tw., are run into the boiler, and when thewhole is melted, the mixture is heated to ebullition,stirring carefully all the time to prevent the rosinadhering to the bottom and sides of the caldron.Should the mass begin to swell out, the heat mustbe decreased. The first boiling should be continuednot more than two or three hours, on account of thefacility with which the combination of the fat andalkali is effected. After six hours’ repose the ex-hausted lye is withdrawn, and fresh substituted, andthe whole is again boiled for three hours or more.This is repeated, and the boilings are thus continued,day after day, until the soap has acquired a properconsistence, which may be ascertained by taking asmall portion, and when cool, squeezing it betweenthe thumb and finger. If hard thin scales areformed, it is finished, and after having boiled briskly,the fire is removed; but if greasy, clammy, and soft,it is not perfect, and it is necessary to add more lyeand reboil. The soap is now to be cooled by addingthree bucketfuls of lye, and after two hours theliquor is withdrawn. Six or eight bucketfuls ofwater are now added, and the whole is again brisklyboiled, stirring constantly until the soap is melted.A little of the boiling paste is now removed on awooden spatula, and if it run clear from the lye,more water is added, and the boiling is continued.If it should not run, too much water has already
been added, and a half pailful of strong solution ofsalt must be thrown in.
The most delicate part of the operation is that ofboiling, and it therefore requires particular attention.When it is complete the soap should, when aportion is taken on a spatula, and the latter is heldin an oblique position, shake and disperse tremu-lously, as would a body of gelatinous consistence.When it acts thus, the fire may be withdrawn, andthe soap considered as finished.
Rosin soap can be prepared from curd soapby adding to it about 25 per cent, of rosin, thenadding 2 to 4 per cent, of carbonate or bicarbo-nate of soda, and 1 or more per cent, of alumor aluminium sulphate ; the whole being boiledwith water till perfect combination is effected. Toprevent the rosin from precipitating, about 2 percent, of dilute sulphuric acid (1 part of acid to 9parts of water) is stirred into the mixture.
In France yellow soap is produced by a similarprocess to that for other kinds of common soap,adding, however, at the commencement of theoperation 15 to 20 per cent, of rosin. Some manu-facturers make a soap with but 5 to 10 per cent,of rosin, and this being lower priced than soapmade entirely of grease, has a more extensive sale,and is preferred especially for domestic purposes.By its property of dissolving more readily in sea-water than other soaps, it is also well adapted foruse on shipboard.
The common brown soap is made in the sameway as described above, but a greater proportion ofrosin is used ; the common kinds of grease, such assoap fat, bone fat, &c., are substituted for the tallow,either wholly or partially.
Rosin soap has a yellow colour when palm oil isused; otherwise it is apt to be brown. It is veryfirm, rough to the touch, and translucent. The bestquality when cut shows a fibrous structure. Italways retains the smell of rosin.
Meinecke's Process .—The soap-pan is constructedwith a still head and cooling worm, in order to con-dense the oil of turpentine which is given off fromthe rosin, which is used in the form of white turpen-tine ; 1000 lbs. of white turpentine are melted inthe kettle by steam with 800 lbs. of tallow or otherfat; and when the mixture reaches 108° Fahr., 800lbs. of caustic soda lye, containing 30 per cent, ofdry soda, are added by slow degrees. The saponi-fication is very rapid at this temperature, the acidsof the rosin and fat being completely neutralizedand converted into liquid melted soap. The essentialoil of turpentine is set free at the same time ; and inorder to promote its evaporation salt or brine is added.The head being then luted on the pan and the wormfitted to it, the mixture is brought to the boil, whensteam and oil of turpentine pass into the worm, andare there condensed. When all the essential oil isdistilled over, the soap is finished in the usual way.
Dunn's Process .—Steam is used to heat the lyes,whilst air is forced below their surface by meansof a pump; the saponification is thereby greatlyaccelerated. Into each of the ordinary boiling