In order to preserve Ether from decomposition, it must be kept in stoppered bottles, nearly full, and in a dark place. The stoppers should be tied over with bladder and luted, or a considerable amount of evaporation will take place, unless the neck of the bottle has been ground with unusual care. After the lapse of some months, probably a certain amount of decomposition, evidenced by the liberation of Iodine on adding Iodide of Potassium, will be found to have taken place. This however is small in amount, and not of a character to injure the fluid. Rectification of Spirits of Wine from Carbonate of Potash.--The object of this operation is to remove a portion of water from the spirit, and so to increase its strength. Alcohol thus purified may be added to Collodion almost to any extent, without producing glutinosity and rottenness of film. The salt termed Carbonate of Potash is a deliquescent salt,--that is, it has a great attraction for water; consequently when Spirits of Wine are agitated with Carbonate of Potash, a portion of water is removed, the salt dissolving in it and forming a dense liquid, which refuses to mix with the Alcohol, and sinks to the bottom. At the expiration of two or three days, if the bottle has been shaken frequently, the action is complete, and the lower stratum of fluid may be drawn off and rejected. _Pure_ Carbonate of Potash is an expensive salt, and a commoner variety may be taken. It should be well dried on a heated metal plate, and powdered, before use. The quantity may be about two ounces to a pint of spirit; or more, if an unusually concentrated Alcohol is required. After the distillation is complete, a fluid is obtained containing about 90 per cent, of absolute Alcohol, the remaining 10 per cent, being water. The specific gravity at 60° Fahrenheit should be from ·815 to ·825; commercial Spirit of Wine being ·836 to ·840. PREPARATION OF THE IODIZING COMPOUNDS IN A STATE OF PURITY. These are the Iodides of Potassium, Ammonium, and Cadmium. The properties of each are more fully described in Part III. a. _The Iodide of Potassium._--Iodide of Potassium, as sold in the shops, is often contaminated with various impurities. The first and most remarkable is _Carbonate of Potash_. When a sample of Iodide of Potassium contains much Carbonate of Potash, it forms small and imperfect crystals, which are strongly alkaline to test-paper, and become moist on exposure to the air, from the deliquescent nature of the Alkaline Carbonate. _Sulphate of Potash_ is also a common impurity; it may be detected by Chloride of Barium.