692 MEDICAL JURISPRUDENCE If the slide is left aside for about fifteen minutes much larger crystals are formed and are easily seen under the microscope. If the drops of cocaine and potassium permanganate solutions are mixed gently and without rubbing and the slide is left uncovered and allowed to evaporate almost to dryness, the same crystals but of a very large size are formed. Fig. 190.— Microphotograph of Cocaine crystals X 150. (K.B. Dr. N. J. Vazifdar.) (Obtained by gold chloride test with cocaine solution, 1 in 500.) If the solution of cocaine is very weak, rubbing on the slide helps to form the crystals within a minute, otherwise longer time is required, This modification of Hankin's method is useful in obtaining the crystals quickly in a dilute solution of cocaine and in developing larger crystals in stronger solutions in a shorter time. 4. Gold Chloride Test. — A 5 per cent solution of gold chloride in distilled water gives a precipitate with a solution containing cocaine. The precipitate is at first amorphous, but rapidly becomes crystalline. Viewed under the microscope, the crystals are found to be delicate rosettes, or long rods resembling fern-fronds, generally with a stellate arrangement. This is a delicate test and a few crystals are formed even with a solution of 1 in 20,000. Gold chloride solution also gives a crystalline precipitate with novo- caine, but the novocaine gold chloride compound is soluble in dilute hydro- chloric acid, while the cocaine gold chloride compound is insoluble in the same acid. Bagchi and his collaborators 44 have made use of this fact in devising a method of carrying out the determination of a small quantity of cocaine in a sample adulterated with novocaine. 44 Med. Go*., Jan. 1939, p. 29.