The detection of ~foreign starch~ is possible only by use of the microscope; by means of standard preparations an approximate estimate may be made as to the amount and kind of meal added.[207] The examination of starch is especially facilitated by H. Leffmann and W. Beam’s[208] centrifugal method: the sample suspended in water is subjected to rotation for a short time in the centrifugal apparatus. The presence of foreign starch is shown by a white layer in the resulting sediment. This layer can be collected and microscopically examined for foreign starch and husk. In the case of cacao preparations, it is always well to distinguish between unimportant traces and quantities that justify objection.[209] [Illustration: Fig. 102. _A._ Silver membrane with the hairs (Mitscherlich particles) _tr_, and the crystals _f_ and _K_. _B._ Cocoa powder: _c_ Cotyledon tissue with cells of fat and colouring matter, _p_ shell parenchyma, _sp_ speriods, _d_ layer of dry cells.] A means of detecting ~tragacanth~ in cacao preparations, has lately been described by Welmans[210]. 5 grammes of the cacao preparation are to be mixed with sufficient dilute sulphuric acid (1: 3) to form a thick pulp, then with 10 drops of solution of iodine (in potassium iodide) and some glycerine. A portion of the mixture is examined under the microscope (enlarged 160 times). The entire field of view now appears to be thickly sown with countless blue dots, some globular, others irregular, among which are especially to be noticed the large tragacanth cells, resembling potato starch, which are not seen in cocoa powder that is free from tragacanth, when similarly prepared as an object; the small blue dots, due to cacao starch, are visible only in the densely occupied portions. An admixture of the ~carob~, which has been seldom observed, can be easily recognised under the microscope by the characteristic reddish wrinkled tubes of the fruit pulp, which are also coloured violet by treatment with a warm solution of caustic potash. The presence of ~earth-nut or earth-nut cake~ can be detected by the aid of the microscope on treatment with chloral hydrate, by the characteristic saw toothed epidermis cells of the husk of arachis seed. ~Hazelnut and walnut pulp~, so far as they are to be met with in cacao preparations, can be distinguished under the microscope by shreds of the tissue of the seed husks, in which broad streaks of spiral vessels, lying close on one another, are distinctly prominent. If in addition the woody fruit shell be admixed, it can be detected by the great number of cells. B. Definitions of Cacao Preparations.