The process of purification is thus far one of fractional distillation combined with chemical washing. In fact, all the processes of purification to which these oils are submitted are essentially of the same character. The principle of fractional distillation has already been explained sufficiently for our present purpose. The process of washing a liquid may appear mysterious to the uninitiated, but in principle it is extremely simple. If we pour some water into a bottle, and then add some liquid which does not mix with the water--say paraffin oil--the two liquids form distinct layers, the one floating on the other. On shaking the bottle so as to mix the contents, the two liquids form a homogeneous mixture at first, but on standing for a short time separation into two layers again takes place. Now if there was present in the paraffin oil some substance soluble in the oil, but more soluble in water, such as alcohol, we should by the operation described wash the alcohol out of the oil, and when the liquids separated into layers after agitation the watery layer would contain the alcohol. By drawing off the oil or the water the former would then be obtained free from alcohol--it would have been "washed." This operation is precisely what the manufacturer does on a large scale with the coal-tar oils. These oils contain certain impurities of which some are of an acid character and dissolve in alkalies, while others are basic and dissolve in acid. The oil is therefore agitated in a suitable vessel provided with mechanical stirring gear with an aqueous solution of caustic soda, and after separation into layers the alkaline solution retaining the acid impurities is drawn off. Then the oil may be washed with water in the same way to remove the lingering traces of alkali, and then with acid--sulphuric acid or oil of vitriol--which dissolves out basic impurities and certain hydrocarbons not belonging to the benzene series which it is desirable to get rid of. A final washing with water removes any acid that may be retained by the oil.