An adulteration with _fat oil_ (poppy oil, castor oil) may be recognized as follows: Place a drop of the suspected oil upon blotting paper and expose it to the heat of the water bath. If it evaporates completely and no stain is perceptible, the oil is pure. But frequently a transparent stain remains with old oils without their being adulterated, which is due to the resin formed by the absorption of oxygen and remaining dissolved in the oil. In this case a transparent ring is generally formed by the concentration of the resin on the edges of the stain. If no tangible results are obtained by this test, pour a few cubic centimeters of the oil upon a watch-crystal and heat it very slowly upon a piece of sheet-iron, until all the odor has disappeared. If the watch-crystal becomes empty in a short time, nothing but volatile oil was present; but if a viscous residue remains, this may consist either of fatty oil or resin, or of both. Treat the residue with strong alcohol; if it dissolves it may be resin or castor oil. Dilute the solution with much water; a white flocculent turbidity indicates resin; the separation of an oily liquid, after standing, castor oil. If the residue remains undissolved, it consists of a fatty oil, generally oil of almond or olive. The presence of castor oil can be accurately determined by bringing the residue from the watch-crystal into a test-tube by means of a glass-rod, and compounding it with a few drops of nitric acid. A strong development of gas takes place, after the cessation of which, solution of carbonate of soda is added as long as there is any sign of effervescence. If the added oil was castor oil, the contents of the test-tube will show a peculiar odor due to œnanthylic acid formed by the action of nitric acid upon castor oil. Another method of establishing the presence of fat oil consists in mixing the suspected oil with eight times its quantity of 90 per cent. alcohol (specific gravity 0.823). If the oil is unadulterated a clear solution is formed; if it contains fat oil, the latter remains undissolved. The presence of castor oil, which of the fat oils is chiefly used for adulteration, is, however, not shown by this method, it being also soluble in alcohol. A permanent stain upon the paper may, however, also be formed by fresh oils obtained by expression from the respective parts of the plant. Thus, lemon oil obtained by expression from the peel, and which has a far more agreeable odor than that produced by distillation, always leaves behind a slight grease-stain.