Rectified Ether of 750° is not to be depended on, inasmuch as the specific gravity is often made up by adding water instead of alcohol. Methylated Ether should be used only when economy is an object, as it is prone to acidity and less certain in its properties. Some of the qualities which render Ether unfit for Photographic purposes, are as follows:--a peculiar and disagreeable smell, either of some essential oil, or of Acetic Ether; an acid reaction to test-paper; a property of turning alcoholic solution of Iodide of Potassium brown with unusual rapidity; an alkaline reaction to test-paper; a high specific gravity, from superabundance of Alcohol and water. The Ether which has been both washed and redistilled is always the most uniform in composition, and especially so if the second distillation be conducted from Quicklime, Carbonate of Potash, or Caustic Potash. These Alkaline substances retain the impurities, which are often of an acid nature, and leave the Ether in a fit state for use. The redistillation of Ether is a simple process: in dealing with this fluid however the greatest caution must be exercised, on account of its inflammable nature. Even in pouring Ether from one bottle into another, if a light of any kind be near, the vapour is apt to take fire; and severe injuries have been occasioned from this cause. _Purification of Ether by redistillation from a caustic or carbonated alkali._--Take ordinary rectified Sulphuric Ether, and agitate it with an equal bulk of water to wash out the Alcohol; stand for a few minutes until the contents of the bottle separate into two distinct strata, the lower of which--_id est_, the watery stratum--is to be drawn off and rejected. Then introduce Caustic Potash, finely powdered, in the proportion of about one ounce to a pint of the washed Ether; shake the bottle again many times, in order that the water--a small portion of which is still present in solution in the Ether--may be thoroughly absorbed. Afterwards set aside for twenty-four hours (not longer, or the Potash may begin to decompose the Ether), when it will probably be observed that the liquid has become yellow, and that a flocculent deposit has formed in small quantity. Transfer to a retort of moderate capacity, supported in a saucepan of warm water, and properly connected with a condenser. On applying a gentle heat, the Ether distils over quietly, and condenses with very little loss; care must be taken that none of the alkaline liquid contained in the body of the retort finds its way, by projection or otherwise, into the neck, so as to run down and contaminate the distilled fluid. A more economical plan of purifying Ether is, without previous washing with water, to agitate with Carbonate of Potash or with Quicklime, and redistil at a moderate temperature.