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troduced. The mixture is then boiled for a few hours, until nearly the whole of the free alkali has been taken up, and the soap again separated from the liquor, when the fire is again withdrawn, the mixture cooled, and the spent ley pumped off. These operations are several times repeated with leys gradually increasing in strength to 36° Tw. (1·180), until the soap tastes slightly alkaline, feels dry and not oily to the fingers, and presents certain appearances indicative of a sufficient action of the alkali; in general, two or three boils are given daily for as many days.* The boiling with ley several times successively serves not only to complete the saponification, but to wash and purify the soap.

In Germany, where potash is just as accessible as soda, hard soap is made by a process very different from that followed in this country. The oil or tallow is first completely saponified by means of potash ley, and the potash soap thus formed is decomposed by adding a considerable quantity of common salt, which gives rise to the formation by a double decomposition of a soap of soda and chloride of potassium. The change is simply a transposition of the potassium in the soap and the sodium in the common salt. As chloride of potassium does not cause the separation of soda soap from its aqueous solution like common salt, it is necessary to apply considerably more of the latter than is necessary for the mere decomposition. The above process was formerly pursued in this country, but the high price of potash, and the low price

* When kelp was used in the soap manufacture instead of soda-ash, and weaker leys employed than at present, three boils were frequently given for six successive days.

and good quality of British soda-ash, have led to its entire rejection.

When the soap is properly formed, and completely separated from the ley, it is removed from the soappan, being in a pasty state, conveyed in buckets to the "frame-room," and poured into the rectangular cisterns called the "frames," or "sesses," in the soaperies of Liverpool, to solidify. The frames in common use for white and mottled soap, and also for yellow soap until recently, are made of wood, and consist of a pile of equal rectangular frames measuring internally forty-five inches by fifteen inches, the thickness or height of each being about four inches. These frames being made very smooth, so as to fit closely together, are piled one upon another to the height of from five to twelve feet, thus forming a square well often large enough to contain more than two tons of The frames are bound tightly together by means of iron screw-rods. For cooling and solidifying yellow soap, frame-moulds made of cast-iron have of late been introduced, such frames being composed of five rectangular plates (one for the bottom, two for the ends, and two for the sides), so arranged and fastened together as to form a well of the same length and breadth as the wooden frames, and about five feet in height. These frames are put together and taken abroad with more facility than the wooden frames, and the good conducting power of the iron considerably accelerates the cooling of the soap. internal length and breadth of the frame are regulated by the Excise, but the depth is left to the convenience of the manufacturer, excepting that it must not be less than forty-five inches.

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As soon as the soap is put into the frames, it is mixed with a small quantity of water, and well agitated with a wooden pole called the "crutch" until nearly cold. If allowed to solidify in the state in which it is withdrawn from the copper, the soap would have a rough granulated texture, and be very hard (if made from tallow and soda only); but it acquires a finer grain, and becomes softer, by being agitated with a little water.

When the mass of soap has solidified and become cold, the iron screw-rods which bind the frames together are removed, and each frame is lifted off the soap, which is left as a compact mass of the size of the interior of the frame. This mass is then cut by means of a wire into slabs of two or three inches in thickness, which are subdivided by a machine into the bars in which the soap is sold. The bars are piled away one upon another crossways, in the form of a wall, interstices being left for the circulation of air; in which state they are allowed to remain for a short time, to allow of the exudation of the saline solution with which the soap is still impregnated to a small extent.

As the exhausted leys, which are pumped out of the soap-copper, are not wholly deprived of their free alkali, and contain sulphate of soda besides, they should be evaporated to dryness and the residue be calcined with lime-stone and small coal, in order to obtain the alkali in the state of carbonate.

Mottled Soap.-Mottled or marbled soap is made from the same kind of materials as white soap, but the tallow for the former is selected with less care

than that for the latter, and the heterogeneous fatty substance known as "kitchen-stuff" generally forms one of the materials for mottled soap. The process of saponification is conducted in just the same manner for mottled as for white soap, the principal difference consisting merely in the relative time allowed for the separation of the soap from the last ley, before it is transferred to the frames; mottled soap being removed earlier than white soap. The brown

veins in mottled soap are due to the presence of exceedingly minute traces of sulphuret of iron derived from the last service of ley, in which the sulphuret was dissolved, probably in combination with alkaline sulphuret, for leys containing sulphuret of sodium yield far more iron to the soap than those which contain none. While the soap is in the frames, the ley diffused through it unequally, slowly separates as the soap cools, and leaves in its track the brownish veins. A perfectly colourless ley, apparently free from every trace of iron, may, nevertheless, contain quite sufficient to produce mottling, if allowed to separate from the soap slowly. In some soaperies the mottling is produced by sprinkling on the surface of the soap, after being boiled with the last service of ley, a small quantity of very strong and sulphuretted ley, which produces the mottling by filtering slowly through the mass of soap.

That the colour of the veins of mottled soap is due to iron, seems evident from the fact that the ash which remains after the calcination of the brown parts of such soap affords to reagents a sensibly larger quantity of iron than the ash of the white parts of the same piece of soap. The iron is, no doubt, principally derived from the rust on the cisterns in which the leys are made. In France, the mottling is produced by adding

Silicated Soap.-The true silicated soap, made according to Mr. Sheridan's patent, consists of a mixture of silicate of soda with hard soap. The silicate of soda is prepared by boiling the powder of flints in a strong caustic ley, until the compound attains about twice the density of water. When of

that strength, it contains, according to Dr. Ure, about thirty-five parts of silica, and forty-six of hydrate of soda, in one hundred parts. This compound is added to hard soap in a liquid or gelatinous state, and the mixture is well triturated in an iron pan, and then conveyed to the frames to solidify. The detergent properties of the soap thus made are very considerable. But the soap now sold under the above name is generally made by mixing with the pasty soap while in the frames a quantity of china clay, previously well rubbed up with a little caustic ley. As the clay possesses, of itself, no detergent property, its addition to the soap can only be regarded as an adulteration. The soap of English manufacture, which paid duty in 1842 as silicated soap, amounted to nine hundred and sixty tons. The whole of this quantity, however, might not have been silicated, for any kind of soap which possesses a greater density than 1.050 is considered by the Excise to be silicated, though it may not contain a particle of silica. This soap generally contains about one-fifth of its weight of silica or china clay.

Yellow Soap.-The essential difference between yellow soap and all other kinds is due to the pre

to the soap, during the boiling, a very small quantity of solution of sulphate of iron (copperas).

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