The solution can be examined as subsequently directed for picrotoxine; the presence of aloes is best recognised by the characteristic saffron-like odour possessed by this body. The syrup which remains after the successive treatments with benzol and amylic alcohol is next freed from any remaining traces of the latter compound by means of blotting-paper, and then thoroughly agitated with anhydrous ether, which is afterwards removed and allowed to spontaneously evaporate. If the residue now obtained exhibits a wormwood-like aroma, and gives a reddish yellow solution, which rapidly changes to a deep blue when treated with concentrated sulphuric acid, _absinthine_ is present. The syrup insoluble in ether may still contain quassine, gentipicrine, and menyanthine, and the presence of any of these bodies is indicated if it possesses a bitter taste, since the bitter principle of hops would have been removed by the foregoing treatment with solvents. The syrup is dissolved in a little warm water, the solution filtered and divided into two portions. To one a concentrated ammoniacal solution of silver nitrate is added, and the mixture heated: if it remains clear, quassine is probably present; the formation of a metallic mirror points to the presence of either gentipicrine or menyanthine. A second portion of the aqueous solution is cautiously evaporated in a small porcelain capsule, and a few drops of strong sulphuric acid are added to the residue: if no change takes place in the cold, but upon applying heat a carmine-red coloration appears, _gentipicrine_ is present; if a yellowish brown colour, which afterwards changes to a violet, is produced, the presence of _menyanthine_ is probable.[81] Picric acid can be detected by means of the following tests:-- 1. Upon shaking pure beer with animal charcoal, it becomes decolorised, whereas beer containing picric acid retains a lemon-yellow colour after this treatment. 2. The bitter taste of normal beer is removed by treatment with a little plumbic diacetate and filtering, which is not the case with the flavour imparted by the use of picric acid. 3. Unbleached wool or pure flannel will acquire a decided yellow colour if boiled for a short time in beer adulterated with picric acid, and afterwards washed. 4. Upon agitating 20 c.c. of the suspected beer in a test-tube with 10 c.c. of amylic alcohol, allowing the mixture to remain at rest, and then removing the amylic alcohol, a solution is obtained which contains any picric acid present in the sample treated. It is evaporated to dryness, the residue dissolved in a little warm distilled water, and the aqueous solution submitted to the following tests:-- (_a_) To one portion a concentrated solution of potassium cyanide is added; in presence of picric acid, a blood-red colour is produced, due to the formation of iso-purpuric acid.