From the data obtained by estimating the "titre," iodine number, and saponification equivalent of the mixed fatty and rosin acids, and the rosin content, a fairly good idea of the constitution of the soap may be deduced. The titre, iodine number, and saponification equivalent are determined in exactly the same manner as described under Fats and Oils. The presence of rosin may be detected by the Liebermann-Storch reaction, which consists in dissolving a small quantity of the fatty acids in acetic anhydride, and adding to a few drops of this solution 1 drop of 50 per cent. sulphuric acid. A violet coloration is produced with rosin acids. The amount of rosin may be estimated by the method devised by Twitchell (_Journ. Soc. Chem. Ind._, 1891, 804) which is carried out thus:-- Two grammes of the mixed fatty and rosin acids are dissolved in 20 c.c. absolute alcohol, and dry hydrochloric acid gas passed through until no more is absorbed, the flask being kept cool by means of cold water to prevent the rosin acids being acted upon. The flask, after disconnecting, is allowed to stand one hour to ensure complete combination, when its contents are transferred to a Philips' beaker, well washed out with water so that the volume is increased about five times, and boiled until the acid solution is clear, a fragment of granulated zinc being added to prevent bumping. The heat is removed, and the liquid allowed to cool, when it is poured into a separator, and the beaker thoroughly rinsed out with ether. After shaking, the acid liquor is withdrawn, and the ethereal layer washed with water until free from acid. Fifty c.c. neutral alcohol are added, and the solution titrated with N/1 KOH or NaOH solution, the percentage of rosin being calculated from its combining weight. Twitchell suggests 346 as the combining weight of rosin, but 330 is a closer approximation. The method may be also carried out gravimetrically, in which case petroleum ether, boiling at 74 deg. C. is used for washing out the beaker into the separator. The acid liquor is run off, and the petroleum ether layer washed first with water and then with a solution of 1/2 gramme KOH and 5 c.c. alcohol in 50 c.c. water, and agitated. The rosin is thus saponified and separated. The resinate solution is withdrawn, acidified, and the resin acids collected, dried and weighed. _Halphen's Reaction._--This is a special test to determine the presence or absence of cotton-seed oil fatty acids in mixtures. Equal parts of the fatty acids, amyl alcohol, and a 1 per cent. solution of sulphur in carbon bisulphide, are heated in a test-tube placed in a water-bath until effervescence ceases, then in boiling brine for one hour or longer when only small quantities are present. The presence of cotton-seed oil is denoted by a pink coloration. The reaction is rendered much more rapid, according to Rupp (_Z. Untersuch. Nahr. Genussm._, 1907, 13, 74), by heating in a stoppered flask.