Where the soap under examination contains mineral matter, the separated fatty acids may be dissolved in ether. This is best performed in an elongated, graduated, stoppered tube, the total volume of the ether, after subsidence, carefully read, and an aliquot part taken and evaporated to dryness in a tared flask, which is placed in the oven at 100 deg. C. until the weight is constant. In a complete analysis, the figure for fatty acids should be converted into terms of fatty anhydrides by multiplying by the factor 0.9875. In this test the resin acids contained in the soap are returned as fatty acids, but the former can be estimated, as described later, and deducted from the total. _Total Alkali._--The best method is to incinerate 5 grammes of the soap in a platinum dish, dissolve the residue in water, boil and filter, making the volume of filtrate up to 250 c.c., the solution being reserved for the subsequent determination of salt, silicates, and sulphates, as detailed below. Fifty c.c. of the solution are titrated with N/1 acid, to methyl orange, and the result expressed in terms of Na_{2}O. Number of c.c. required x 0.031 x 100 = per cent. Na_{2}O. The total alkali may also be estimated in the filtrate from the determination of fatty acids, if the acid used for decomposing the soap solution has been measured and its strength known, by titrating back the excess of acid with normal soda solution, when the difference will equal the amount of total alkali in the quantity taken. The total alkali is usually expressed in the case of hard soaps as Na_{2}O, and in soft soaps as K_{2}O. _Free caustic alkali_ is estimated by dissolving 2 grammes of the soap, in neutral pure alcohol, with gentle heat, filtering, well washing the filter with hot neutral spirit, and titrating the filtrate with N/10 acid, to phenol-phthalein. Number of c.c. required x 0.0031 x 50 = per cent. free alkali Na_{2}O, as caustic. _Free Carbonated Alkali._--The residue on the filter paper from the above determination is washed with hot water, and the aqueous filtrate titrated with N/10 acid, using methyl orange as indicator. The result is generally expressed in terms of Na_{2}O. Number of c.c. required x 0.0031 x 50 = per cent. free alkali Na_{2}O, as carbonate. _Free Alkali._--Some analysts determine the alkalinity to phenol-phthalein of the alcoholic soap solution without filtering, and express it as free alkali (caustic, carbonates, or any salt having an alkaline reaction). _Combined Alkali._--The difference between total alkali and free alkali (caustic and carbonate together) represents the alkali combined with fatty acids. This figure may also be directly determined by titrating, with N/2 acid, the alcoholic solution of soap after the free caustic estimation, using lacmoid as indicator. The potash and soda in soaps may be separated by the method described for the estimation of potassium in _Pearl ash_ (page 126).