IV. _Hager's guaiacum reaction_[3] serves for the detection of oil of turpentine in a volatile oil. By pouring upon as much guaiacum, freshly powdered, as will lie upon the point of a small knife, in a test-tube 1 cubic centimeter (25 drops) of spike oil, and heating nearly to boiling over a petroleum lamp, the oil after being removed from the flame and allowing the undissolved resin to settle, shows a _yellow_ color. By now pouring upon an equal quantity of guaiacum in another test-tube 25 drops of spike oil and 5 drops of rectified oil of t from the flame shows a _dark violet color_. Various other oils behave in the same manner as spike oil, and hence a content of oil of turpentine can be readily detected in them. Other oils do not exhibit this behavior; but this can be remedied by adding, in testing for oil of turpentine, a few drops of an oil of the first class. [3] Hager, Chemische Reactionen zur Nachweise des Terpentinoels in den aetherischen Oelen, etc. Berlin, 1885. The guaiacum reaction is an ozone reaction and with reference to this, the volatile oils may be divided into three classes:-- _a. Oils inclining to the formation of ozone._--Foremost of these is oil of turpentine, especially when rectified. Oils of tansy, rue, mint, juniper, zedoary, etc., show considerably less inclination. _b. Oils which, especially when heated, directly incite the oil of turpentine to form ozone, and to color guaiacum violet or blue._--Such oils are many kinds of oil of citronella, oils of spike, calamus, cedar, etc. _c. Oils with a content of oil of turpentine, which remain indifferent towards guaiacum._--To such oils, if to be tested for oil of turpentine, with the assistance of the guaiacum reaction, a few drops of an oil of the second class have to be added. V. _Hübl's iodine method._--Mr. C. Barenthin has applied Hübl's iodine method for fixed oils to the examination of volatile oils. He uses the following solutions:-- 1. Fifty grammes iodine and 60 grammes of mercuric chloride in a liter of alcohol freed from fusel oil, and let stand for 12 hours. 2. Twenty-four grammes of hyposulphite of sodium in a liter of water.