1. A portion of the sample is agitated with castor oil and filtered. In case turmeric is present, the filtrate will exhibit a marked greenish fluorescence. 2. Upon treating the suspected sample with ammonium hydroxide, an orange-red colour is produced in presence of turmeric. Or, the mustard is boiled with methylic alcohol, the extract filtered, evaporated to dryness, and the residue treated with hydrochloric acid; if turmeric be present, an orange-red coloration takes place, which changes to a bluish-green upon adding an excess of sodium hydroxide. In addition to the above qualitative tests, valuable indications regarding the purity of mustard are to be obtained by the determination of the proportions of fixed oil, sulphur, and ash contained in the sample under examination. _Fixed Oil._--The amount of fixed oil is estimated by digesting a weighed portion of the mustard with ether in a closed vessel, filtering, and determining the weight of the residue left upon evaporating the ethereal solution to dryness over the water-bath. The oil possesses a specific gravity ranging from 0·915 to 0·920. The percentage of fixed oil in pure mustard is very considerable (usually over 34 per cent.), whereas the substances commonly added contain but a very small quantity. In case wheaten flour has been employed as an adulterant, the proportion of pure mustard (_x_) in a mixed sample, can be approximately calculated by the following formulæ, in which _y_ is the fixed amount of oil contained.[140] (33·9_x_) / 100 + 1·2(100 - _x_) / 100 = _y_, 36·7_x_ / 100 + 2·(100 - _x_) / 100 = _y_. In the absence of flour, a low percentage of fixed oil indicates the presence of exhausted mustard cake. _Sulphur._--Blyth determines the total sulphur by oxidation with fuming nitric acid, diluting the liquid considerably with water, filtering and precipitating the sulphates formed by means of barium chloride. The proportion of sulphates (in terms of barium sulphate) found in the ash is to be deducted from the weight of the precipitate obtained; the remainder, multiplied by 0·1373, gives the amount of sulphur present in organic combination, and, as the quantity contained in this form in mustard is far greater than in any of the substances employed for its adulteration, the estimation is frequently very useful. _Ash._--The amount of ash is determined in the usual manner, _i. e._ by the incineration of a weighed portion in a platinum capsule. Genuine mustard contains about 5 per cent. of ash, of which nearly 1 per cent. is soluble in water. In presence of inorganic impurities, the quantity of ash is naturally increased, while a proportion under 4 per cent. is usually considered an indication of organic admixture. The composition of the ash of mustard seed is given below:--