500 PRACTICAL ORGANIC CHEMISTRY opened bottle are not left standing about too long.) To prevent bro-mine vapour spreading in the laboratory, a lightly fitting plug of cotton-wool, moistened with 1-2 drops of formic acid, is inserted just into the wide part of the receiver K. The latter is then drawn up over the delivery-tube, so that there is a gap of only 1-2 mm. between the lower end of H and the bottom of K, and it is then held in place by a wooden block. The test sample is now weighed out. This is best done by weighing 20-25 mg. into a small tinfoil cup: (the cup is best made by cutting the corners off a small square of tinfoil, and then pressing this circle into a cup by rubbing it into shape round the end of a glass rod). After the material has been introduced, the cup should be gently tapped on a metal block, while held with balance tweezers, to ensure that no particles of the sample are adhering to its outside edge; this also serves to settle the material down on to the floor of the cup. The cup, after
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