Hager's tannin test is very reliable. Bring into a test-tube 5 to 10 drops of the oil to be examined, add a piece of tannin the size of a pea, shake so that the tannin is moistened by the oil, and let the whole stand at a temperature of 59° to 68° F. In most volatile oils tannin is insoluble, and, if the oil is pure, floats for days on the surface without change. If, however, the oil contains alcohol, the tannin absorbs the latter, according to the quantity present, in 3 to 48 hours, and forms with it a more or less transparent, viscous, tough, or smeary mass resembling a soft resin, which settles on the bottom, and adheres so firmly to it, as well as to the sides of the tube, that it cannot be moved by shaking. The mass may be examined as to its consistency with a knitting needle. Traces of moisture in the oil are not detrimental to the test, the tannin mass separating in the form of a hyaline mass only in few oils, and if this mass is tested with the knitting needle it will be found not tough or smeary, but hard, and may sometimes be divided into small grains. With oil of bitter almonds, cassia oil, and some oils of clove, as well as volatile oil containing an acid, the tannin test is not available. The first two oils even dissolve tannin, and large quantities of it, if they contain alcohol. The above-mentioned oils may, however, be rendered fit for the tannin test by mixing them with double their volume of benzine or petroleum-ether, and allowing the mixture to stand for two or three days. If, however, the oils contain much alcohol, the tannin is dissolved. The use of powdered tannin is not advisable, because it generally deposits in a thin layer on the bottom, and its alteration is not so perceptible. If, for practical reasons, a content of 0.5 per cent. anhydrous alcohol might be accepted as permissible in a volatile oil, the tannin test would have to be so modified as to mix 10 drops of the oil with a piece of tannin the size of two peas, and allow the whole to stand for one hour. In this time the above-mentioned content of alcohol would yield no result.