438 PRACTICAL ORGANIC CHEMISTRY always precede that of the molecular weight of a particular solute: the precise value of the constant depends largely on the "personal equation" of the observer,* and the two determinations should therefore always be made by the same observer to ensure identical conditions of manipulation. The capillary tubes used for the melting-point determinations should be both longer and wider than those normally used: they should be at least io cm. long (to decrease the chance of slipping off the thermometer during the determination) and 2-3 mm. in diameter (in order to have sufficient semi-molten material in the tube to observe accurately the process of melting). (1) DETERMINATION OF THE MELTING-POINT OF THE PURE CAMPHOR. Pulverise some pure camphor on a piece of unglazed porcelain using a clean spatula: transfer some of this camphor to a capillary tube and press it firmly down by means of a second capillary tube sealed at the end and fitting snugly into the larger tube. Sufficient camphor
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