476 PRACTICAL ORGANIC CHEMISTRY The polishing process should take between thirty seconds and one minute and the tube should then be placed on a small stand. The time at which polishing ended should be noted. The other tube is then treated similarly and the time at which polishing was completed again noted; the two tubes should always be polished in the same order. The tubes are left on their rack until ten minutes have elapsed since the completion of polishing of the first one; the latter is then transferred to a small aluminium rider which holds it firmly and evenly on the balance pan. This transfer may best be accomplished by using a pair of thin metal tweezers of wide aperture, whose ends have been slightly bent inwards, to grip the tube firmly and easily. The time at which the final counterpoising and reading is made is noted. At the end of the combustion when the tubes have been momentarily opened to the air, repolished exactly as before and are reweighed, their weight will be
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