6. If the sample consists of butter or oleomargarine, and is mixed with about ten parts of glycerine and the emulsion digested with a mixture of equal parts of ether and alcohol, two layers of solution will be produced, without any deposit of solid matter between them; if, however, lard, suet, or starch is present it will become deposited between the layers. It has already been mentioned that butter differs from some of its fatty adulterants in containing a considerable proportion of fatty acids which are soluble in hot water, the acids present in most foreign fats being, on the other hand, almost entirely insoluble. The estimation of the relative amounts of soluble and insoluble acids contained in a fat possesses therefore much importance; indeed, more significance attaches to this determination than to any other. The processes most frequently employed in the quantitative examination of butter fat are those of Koettstorfer, Hehner, and Reichert. Koettstorfer’s method[28] is based upon the fact that, as butter fat contains the fatty acids, having a smaller molecular weight than those present in other fats, it must contain more molecules of acid, and will therefore require a greater amount of an alkali to effect saponication. The process is executed as follows:--One or two grammes of the filtered fat are weighed out in a narrow beaker and heated over a water-bath with about 25 c.c. of one-half normal alcoholic solution of potassium hydroxide. The saponification of the fat is assisted by repeated stirring; when it is completed the beaker is removed from the bath, a few drops of alcoholic phenol-phthaleine added for an indicator, and the excess of potash used titrated back with one-half normal hydrochloric acid. It has been found that pure butter fat requires from 221·4 to 232·4 milligrammes of potassium hydroxide for saponification. The following are the number of milligrammes of alkali necessary for the saponification of one gramme of various other fats:-- mgr. Olive oil 191·8 Rape-seed oil 178·7 Oleomargarine 195·5 Beef tallow 196·5 Lard 195·5 Mutton suet 197·0 Dripping 197·0 Taking 227 milligrammes as the average amount of potassium hydroxide required to saponify one gramme of pure butter fat, the following formula has been suggested for the estimation of the proportion of admixture in a suspected sample:-- (227 - _n_) × 3·17 = _x_, in which _n_ represents the number of milligrammes of potassium hydroxide used, and _x_ the percentage of foreign fat added. In the Paris Municipal Laboratory, 221 milligrammes of K(OH) are regarded as a standard for the saponification of one gramme of genuine butter.