This section is from the book "A Treatise On Therapeutics, And Pharmacology Or Materia Medica Vol1", by George B. Wood. Also available from Amazon: Part 1 and Part 2.
Ether is obtained by the distillation of a mixture of alcohol and sulphuric acid; but, as thus procured, it is impure, containing, besides the pure ether, sulphurous acid, ethereal oil, alcohol, and water. From these it is purified by redistillation from a strong solution of potassa.
Ether consists of the same ingredients as alcohol less one equivalent of hydrogen and one of oxygen, in other words, less one equivalent of water. This equivalent of water is abstracted by the sulphuric acid. Chemists differ as to the precise mode in which the end is attained; but the result is as stated. At the same time, a reaction takes place, by which a very small portion of the sulphuric acid is converted into sulphurous acid, and of the alcohol into heavy oil of wine, or ethereal oil. Hence the presence of these impurities in the product of the first distillation. In the second distillation, the potassa neutralizes the sulphurous acid, and abstracts any water that may be present; the alcohol is retained by the water, and the ethereal oil is either decomposed or remains behind. The ether obtained, however, still contains a little alcohol, from which it is not necessary that it should be entirely freed for certain medical purposes, especially as an external application. But for inhalation, it is important that it should be as pure as possible; and both the U. S. and British Pharmacopoeias give processes for its purification; designating the resulting preparation by names denoting either its greater strength or purity.