Cocoa-nut oil, unfortunately, requires a figure (250 mgr.) considerably above that of butter, and it is quite possible to prepare a mixture of this oil and oleomargarine, that by the foregoing test would show a result almost identical with that afforded by pure butter. Hehner’s process,[29] which is often employed for the determination of the insoluble fatty acids, is as follows:--About 4 grammes of the melted and strained sample are dissolved in 50 c.c. of alcohol, containing two grammes of potassium hydroxide in solution, and the mixture is heated until complete saponification takes place. The alcohol is removed by evaporation, the residue dissolved in 200 c.c. of water, and the fatty acids precipitated by adding dilute sulphuric acid to distinct acid reaction. The fatty acids are next melted by heating the liquid and are then allowed to cool, after which the insoluble acids are poured upon a tared filter and repeatedly washed with hot water until the washings cease to show acidity. The filter and contents are finally cautiously dried and weighed. In genuine butter the proportion of insoluble fatty acids ranges between 86·5 and 87·5 per cent.; it should not be above 88 per cent.[30] Oleomargarine, lard, mutton, beef, and poppy, palm, olive, and almond oils contain about 95·5 per cent. of insoluble acids.[31] The preceding process is also imperfect in not effecting the detection of cocoa-nut oil, which affords only about 86 per cent. of insoluble fatty acids, and although the presence of any considerable proportion of this oil in butter would probably be indicated by the decreased melting point of the admixture, an estimation of the soluble fatty acids is by far the most reliable means for its detection. For this determination Reichert’s method[32] is eminently adapted. In this process advantage is taken of the facts that the amount of soluble acids in a mixture of fat bears a direct relation to the proportion of genuine butter present, and that, if the aqueous solution of a saponified fat is decomposed by an acid and heated to boiling, the greater portion of the soluble acids escape with the watery vapours and can be collected and determined in the distillate. The details of this method are essentially as follows:--2½ grammes of the filtered sample are introduced into an Erlenmayer flask together with 1 gramme of potassium hydroxide and 20 c.c. of dilute (80 per cent.) alcohol, and the mixture is heated over the water-bath until complete saponification is effected, and the alcohol _entirely_ removed. The soap thus formed is dissolved in 50 c.c. of water, and decomposed by adding 20 c.c. of dilute sulphuric acid (1:10). The flask is next connected with a Liebig’s condenser and the contents carefully distilled until 50 c.c. have passed over. The distillate is now freed from any insoluble acids possibly present by filtration; it is then titrated with decinormal soda solution, a few drops of litmus solution being employed as an indicator.