The farina or black and white mustard differs but little in appearance. The brown, however, is slightly darker. The outer colorous epidermis consists of angular plates, or hexagonal tabular cells, with a center of different brilliancy. It swells up and becomes slimy in water, and, therefore, must be observed in glycerine. At the best it requires some manipulation to see it well, and it is far less prominent in the black seed. The next coat, denominated the subepidermal, is not prominent and can only be seen at all easily in the white seed. The third layer is an important one, and in it is found the coloring matter of the brown seed. Its absence is the cause of the lack of color in the white variety. By this layer one is able to tell whether the flour is a mixture of both the black and white seed or if it is derived from one only. Fragments of this layer are common in powdered mustard. It is distinguished by the thick or colorless brown cell walls and their irregular dotted appearance. Between the third and second layers are numerous cells containing some color in the brown seed, but of little importance. Within these comes the important layer, denominated the inner tunic or plasma layer. It separates readily from the other parts of the husk and is often found by itself in the powdered mustard. As its contents are broken up by water or chloral-hydrate, it is necessary to use glycerine or oil in mounting. The cells and their contents of this layer are large and much alike in both the black and white seed. The interior of the mustard seed is made up of small, soft parenchyma cells, containing the oil and the other constituents of the mustard, but without any trace of starch—a fact which makes adulterations easily detected. The peculiar pungency and odor of the black seed are due to an essential or fixed oil, myronic acid, which is developed by the action of cold water (hot water will not answer) on two peculiar chemical substances which it contains, which form a compound, termed by the discoverers myronate of potash, but since called synanthrin, an acid with formula C_{10}H_{19}NS_{2}O_{10}. This acid is converted into the volatile oil of mustard or sulphocyanide of allyl C_{4}H_{5}NS, or C N } S. C_{3}H_{5} } Through the agency of the myrosin, another constituent of brown seed, when the two are brought in contact through the medium of water, we have vegetable albumen, a bitter principle, a little gum and sugar, and a peculiar green substance, cellulose, and mineral water, called sulphocyanide of sinapine.[8] The aqueous extract of yellow mustard seed yields with a solution of ferric chloride a deep, blood-red coloration, which is scarcely perceptible with similar extract of black mustard. The aqueous extract of white mustard acquires a powerful odor of sulphurated hydrogen in a few hours, while that of the black seed smells only of the pungent mustard oil. ----- Footnote 8: Sinapaline sincaline.