If the dilution with water has been allowed to proceed too far, and too fine a fit is produced, which would be denoted by the layer of soap not leaving the trowel, a little caustic lye or brine may be very carefully added and the whole well boiled until the desired condition is obtained. A good pressure of steam is now applied to the pan, causing the contents to swell as high as possible, this greatly facilitating the settling of impurities; steam is then turned off, the pan covered, and the boil allowed to rest for several days. The art of fitting consists in leaving the contents of the pan in such a condition that, on standing, all the impurities precipitate, and the settled soap, containing the correct amount of water, is clear and bright. The above is a general practical outline of the ordinary soap-boiling process. It may be modified or slightly altered according to the fancy of the individual soap-maker or the particular material it is desired to use. Fats and oils not only vary in the amount of alkali they absorb during saponification, but also differ in the strength of the alkali they require. Tallow and palm oil require lye of a density of 15 deg. to 18 deg. Tw. (10 deg. to 12 deg. B.), but cocoa-nut oil alone would not saponify unless the lye was more concentrated, 33 deg. to 42 deg. Tw. (20 deg. to 25 deg. B.). Cotton-seed oil requires weak lyes for saponification, and, being difficult to saponify alone even with prolonged boiling, is generally mixed with animal fat. When fats are mixed together, however, their varying alkali requirements become modified, and once the saponification is begun with weak lye, other materials are induced to take up alkali of a strength with which alone they would not combine. It is considered the best procedure to commence the pasting or saponification with weak lye. In order to economise tank space, it is the general practice to store strong caustic lye (60 deg. to 70 deg. Tw., 33 deg. to 37 deg. B.) and to dilute it as it is being added to the soap-pan by the simultaneous addition of water. Many manufacturers give all their soap a "brine wash" to remove the last traces of glycerine and free the soap from carbonates. This operation takes place prior to "fitting"; sufficient water is added to the boiling soap to "close" it and then brine is run in to "grain" it. After resting, the liquor is withdrawn. Having described the necessary operations in general, we will now consider their application to the preparation of various kinds of hard soap. _Curd Soaps._--Tallow is largely used in the manufacture of white curd soaps, but cocoa-nut oil sometimes enters into their composition.