The furfurol reaction proposed by Schiff[141] appears to be well suited for the detection of carbohydrates. It is made as follows: Xylidin is mixed with an equal volume of glacial acetic acid and the solution treated with some alcohol. Strips of filter paper are then dipped in the solution and dried. When these strips of prepared paper are brought in contact with the most minute portion of furfurol, furoxylidin is formed, C₄H₃OCH(C₈H₈NH₂)₂, producing a beautiful red color. In practice, a small portion of the substance, supposed to contain a carbohydrate, is placed in a test tube and heated with a slight excess of concentrated sulfuric acid. The prepared paper is then placed over the mouth of the test tube so as to be brought into contact with the escaping vapors of furfurol. The furfurol reaction with α-naphthol for some purposes, especially the detection of sugar in urine, is more delicate than the one just described. This reaction was first described by Molisch[142], who, however, did not understand its real nature. The process is carried on as follows: The dilute solution should contain not to exceed from 0.05 to one-tenth per cent of carbohydrates. If stronger, it should be diluted. Place one drop of the liquid in a test tube with two drops of fifteen per cent alcoholic solution of α-naphthol, add carefully one-half cubic centimeter of concentrated sulfuric acid, allowing it to flow under the mixture. The appearance of a violet ring over a greenish fringe indicates the presence of a carbohydrate. If the substance under examination contain more than a trace of nitrogenous matter, this must be removed before the tests above described are applied. If the liquids be mixed by shaking when the violet ring is seen, a carmine tint with a trace of blue is produced. If this be examined with a spectroscope, a small absorption band will be found between D and E, and from F outward the whole spectrum will be observed. One drop of dextrose solution containing 0.05 per cent of sugar gives a distinct reaction by this process. It can be used, therefore, to detect the presence of as little as 0.028 milligram of grape sugar. This test has been found exceedingly delicate in this laboratory, and sufficiently satisfactory without the spectroscopic adjunct. The furfurol reaction is useful in detecting the presence of minute traces of carbohydrates but is of little value in discriminating between the different classes of these bodies. It is not practical here to go into greater detail in the description of qualitive reactions. The analyst, desiring further information, should consult the standard works on sugar chemistry.[143]