852 SOAP 



alternately, till tho whole quantity of oil destined for the pan is introduced. The 

 ebullition is kept up in the gentlest manner possible, and some stronger lye is oc- 

 casionally added, till the workman judges the saponification to be perfect. The 

 boiling becomes progressively less tumultuous, the frothy mass subsides, the paste grows 

 transparent, and it gradually thickens. The operation is considered to bo finished 

 when the paste ceases to affect the tongue with an acrid pungency, when all milkiness 

 and opacity disappear, and when a little of the soap placed to cool upon a glass-plate 

 assumes the proper consistency. 



A peculiar phenomenon may be remarked in the cooling, which affords a good 

 criterion of the quality of the soap. When there is formed around the little patch an 

 opaque zone, a fraction of an inch broad, this is supposed to indicate complete saponifi- 

 cation, and is called the strength ; when it is absent, the soap is said to want its strength. 

 When this zone soon vanishes after being distinctly seen, the soap is said to have false 

 strength. When it occurs in tho best form the soap is perfect, and may be secured in 

 that state by removing tho fire, and then adding some good soap of a previous round 

 to cool it down, and prevent further change by evaporation. 



200 Ibs. of oil require for their saponification, 72 Ibs. of American potash of 

 moderate quality, in lyes at 15 B. ; and the product is 460 Ibs. of well-boiled soap. 



If hempseed oil has not been employed, the soap will have a yellow colour, instead 

 of the green, so much in request on the Continent. This tint is then given by the 

 addition of a little indigo. This dye-stuff is reduced to fine powder, and boiled for 

 some hours in a considerable quantity of water, till the stick with which the water is 

 stirred presents, on withdrawing it, a gilded pellicle over its whole surface. The indigo 

 paste diffused through the liquid, is now ready to be incorporated with the soap in the 

 pan before it stiffens by cooling. 



Estimation of the quantity of water in soap: Take about 1,000 grains of the soap 

 under examination, cut into small and thin slices, not only from the outside, which is 

 always drier, but from the interior of the sample, so that the whole may represent a fair 

 average; mix the mass well together, and of this weigh accurately 100 grains; place 

 it in an 'oven heated to a temperature of 212 Fahr., until it is quite dry, weighing it 

 occasionally until no loss or diminution of weight is observed, the difference between 

 the original and the last weight, tho )oss, indicates, of course, the proportion of water. 

 The loss of water in mottled soap and in soft soap should not be more than 30 to 35 

 per cent. ; in white or yellow soap from 36 to at most 50 per cent. 



If the soap is sulphated, the amount of sulphate employed may be determined by 

 taking 200 grains of the sample, dissolving it in a capsule with boiling water, adding 

 to the boiling solution as much hydrochloric acid as is necessary to render tho liquid 

 strongly acid, and- therefore to decompose the soap entirely throwing the whole in a 

 filter previously ''wetted with water, adding to the filtrate an excess of chloride of 

 barium, washing' thoroughly the white precipitate so produced, igniting and weighing 

 it; every grain of sulphate of baryta thus obtained represents T467 grain of crys- 

 tallised sulphate of soda. 



If the soap contains clay, chalk, silica, dextrine, fsecula, pumice-stone, ochre, plaster, 

 salt, gelatine, &c., dissolve 100 grains of the suspected soap in alcohol, with the help 

 of a gentle heat ; the alcohol will dissolve the soap and leave all these impurities in 

 an insoluble state. Good mottled soap should not leave more than 1 per cent, of 

 insoluble matter, and white or yellow soap still less. All soap to which earthy or 

 siliceous matter has been added is opaque instead of transparent at the edges, as is the 

 case with all genuine or fitted and sulphated soap. The drier the soap, the more 

 transparent' it is. 



Bone-soap, or glue-soap, is recognised by its unpleasant odour of glue and its dark 

 colour, its want of transparency at tho edges ; that made with the fat of the intestines 

 of animals has a disgusting odour of faeces. 



When uncombined silica has been added to soap, its presence 1 may be readily 

 detected by dissolving the suspected soap in alcohol, as before, when the silica will be 

 left in an insoluble state ; but if the silica is in the state of silicate of soda or of 

 potash, it is necessary to proceed as follows : Dissolve a given weight of tho 

 suspected soap in boiling water, and decompose it by the gradual addition of moderately 

 dilute hydrochloric acid, until the liquor is strongly acid ; boil the whole for one or 

 two minutes longer and allow it to cool in order that the fatty acids having separated 

 and become hard, may be removed. Evaporate the acid liquor to perfect dryness, and 

 the perfectly dry mass treated with boiling water will leave an insoluble residue 

 which may bo identified as silica by its grittiness, which is recognised by rubbing it in 

 the capsule with a glass rod. This white residue should then be collected on a filter, 

 washed, dried, ignited, and weighed. 



The proportion of alkali (potash or soda) may be easily determined by an alkali- 

 metrical assay as follows : 



