SOAP.
335
lent or 564 parts of stearine are converted into one equivalent or 527parts of anhydrous stearic acid, and one equivalent or 37 parts of anhy-drous glycerine ; the stearic acid unites with the alkali to form soap.The following diagram illustrates these changes :—
REAGENTS.
1 eq.Stearine564
73 eq. Carbon 43870 eq. Hydrog. 707 eq. Oxygen 56
' 70 eq. Carb. 420
s
^ 3 eq. Carb.^ 67 eq. Hydr.) 3 eq. Hydr.c 5 eq. Oxyg.^ 2 eq. Oxyg.
1 eq. Anhyd. Glycerine= 37
In the conversion of resin into soap the phenomena are different.Resins consist of one or more acids, which combine with alkalies to formresinous salts or soaps. Thus ordinary yellow resin (or rosin) consistsof two acids, called respectively pinic and silvic acids; and a sodasoap made of this substance would, therefore, be a mixture of pinate andsilvate of soda.
Properties. —The consistence, colour, odour, and sp. gr. of soapvary in the different kinds. The taste of all is slightly, alkaline. Allthe alkaline soaps are soluble both in water and alcohol. The substancecalled transparent soap is prepared by evaporating an alcoholic solutionof pure soap. When heated soap fuses, swells up, and is decomposed,leaving a residuum of charcoal and alkaline carbonate. Most of the acidsdecompose soap : they unite with the alkaline base and separate the fattyacids. The earthy salts (as sulphate of lime, sulphate of magnesia, alum,&c.) also decompose soap : the fatty acids unite with the earth to forman insoluble earthy soap, while the alkali of the soap combines with theacid of the salt. The hardness of sea, spring, and well water, depends onthe earthy salts (principally sulphate of lime) which decompose soap :hence tincture of soap may be used as a test of the hardness or softnessof common waters. The metallic salts also decompose soap, and giverise to metalline soaps.
Olive oil soda soap may be either white or marbled, as before stated.When pure it has very little odour. It is hard, but in the fresh state maybe easily worked or kneaded between the fingers : by keeping in warmair it becomes dry and pulverizable. It should not feel greasy, have arancid odour, communicate an oily stain to paper, nor be covered with asaline efflorescence; but should dissolve completely and readily in bothwater and alcohol. The marbling, as I have before stated, is an impurity.
Animal oil potash soap (common soft soap) is of a brownish or yel-lowish colour; transparent, but interspersed with white specks, owing tothe admixture of tallow.
Characteristics. —Soap may be partly recognised by its physicalproperties, especially by its feel, which is so well known that it is usuallycalled soapy. The solubility of soap in water and alcohol is an impor-tant character, as well as its detergent quality, which depends on itspower of rendering fatty and other matters soluble in water. The effectof heat on it also deserves notice : if the carbonaceous residuum bedigested in weak hydrochloric acid, and the solution filtered and concen-trated by evaporation, the nature of the alkaline base may be ascertainedby applying the tests for potash and soda before mentioned (pp. 274 and310). Lastly, the action of acids and earthy and metallic salts on asolution of soap, as already noticed, serves to recognise soap.