by distilling once or twice over fresh quicklime in the manner described. Purified methylated spirit may generally replace the more expensive ethyl alcohol as a solvent.1 FIG. 2. 2. PURIFICATION OF ETHER. Pure commercial ether, prepared from ethyl alcohol (p. 100) and free from water and alcohol, has no action upon sodium. Its specific gravity at 15° is 702. If there is a slight evolution of hydrogen on 1 COMMERCIAL METHYL ALCOHOL, obtained by purifying wood spirit, is now prepared nearly free from acetone, and, after purification with potash and lime, may also replace ethyl alcohol as a solvent. introducing a small piece of sodium, on adding more sodium the action soon ceases and the ether is then anhydrous. If, on the other hand, the action is brisk, the ether must be left in contact with small pieces of fused calcium chloride for twenty-four hours, poured off, and a few thin slices of sodium introduced. The vessel is then closed by a cork, through which an open chloride of calcium tube is inserted to allow any hydrogen to escape and to prevent absorption of moisture. Methylated ether, prepared from methylated spirit by the same process as the ethyl ether, consists of a mixture of diethyl, ethyl methyl, and dimethyl ethers, and contains usually alcohol and water. For extracting substances from aqueous solutions the ether may be employed without further purification. To free it from alcohol it is shaken up repeatedly with small quantities of water. The water dissolves out the alcohol and forms a layer at the bottom of the liquid, and is drawn off by means of a separating funnel. The ether, separated as carefully as possible from water in this way, is dehydrated over fused calcium chloride, and sodium, as described above. It should not give the iodoform reaction. (See Appendix.) N.B.-In cases where ether is used great care should be taken, that no flame is in the neighbourhood of the liquid. 3. PURIFICATION OF BENZENÉ. Pure commercial benzene, obtained from coal-tar naphtha, should distil within one degree (80°—81°) and solidify completely when cooled to o°. Other tests are as follows: Shaken with concentrated sulphuric acid for a few minutes, the acid should not darken, and a drop of bromine water should not be immediately decolorised. A single distillation over a few small pieces of sodium which absorb any traces of water, is usually a sufficient purification. If the benzene impart a brown or black colour to sulphuric acid it must be repeatedly shaken with about 20 per cent. of the acid until the latter becomes only slightly yellow on standing. This is done in a stoppered separating funnel, and after shaking for a minute the mixture is allowed to settle and the lower layer of acid drawn off. The benzene is then shaken two or three times with water to free it from acid, carefully separated from the aqueous layer and allowed to remain in contact with pieces of fused calcium chloride until the liquid becomes clear. It may then be frozen in ice, and any liquid (carbon bisulphide, paraffins) carefully drained off and the benzene finally distilled over sodium. Commercial 50 per cent. and 90 per cent. benzene are mixtures of benzene and larger or smaller quantities of its higher boiling homologues, viz. toluene (110°) and the xylenes (137°—143°). The benzene may be separated by fractional distillation. It is often possible to separate almost completely by a single distillation two liquids occurring together in a mixture when their boiling-points lie widely apart. The more volatile liquid first passes over, the temperature suddenly rises, and the higher boiling liquid distils. Thus, in the separation of ethyl benzene (p. 16 from its ethereal solution, the ether (B. P. 35°) distils off almost completely at the temperature of the water-bath, leaving behind the ethyl benzene (B. P. 134°). It is otherwise when a liquid consists of a mixture of bodies boiling at temperatures not very far removed from one another, especially in the case of homologous compounds such as occur in petroleum and coal-tar naphtha. One distillation suffices only to produce very partial separation of the different substances, a portion of the less volatile liquid being carried over in the first distillate together with the more volatile body, the temperature gradually rising throughout the distillation. In order to effect separation of the several substances in a case of this kind recourse is had to the method of fractional distillation. The liquid is distilled in a round-bottomed flask over wire-gauze or, better, in a fusible metal bath with a thermometer so placed that the temperature of the vapour of the boiling liquid is indicated, i.e. with the bulb just below the side tube of the fractionating column (Fig. 3). As the temperature gradually rises the distillate, which passes over between every 3o, 5o, or 10°, as the case may require, is collected in separate flasks. These separate portions are called. fractions. Each of these fractions when submitted to a second distillation is usually found to begin to boil at a temperature lower than that indicated in the first instance, so that it is possible in this way to subdivide FIG. 3. Distilling apparatus with Würtz's fractionating column. each fraction into a portion of lower boiling-point, and a portion of higher boiling-point, and the process is continued until certain of the fractions have a nearly constant boiling-point. If the vapour, which rises from the boiling liquid to be fractionated, undergoes slow cooling before reaching the condenser, the less |