Common salt exists abundantly in nature, both in the form of solid rock-salt and dissolved in the waters of the ocean. _Properties of the pure Salt._--Fusible without decomposition at low redness, but sublimes at higher temperatures; the melted salt concretes into a hard white mass on cooling. Nearly insoluble in absolute alcohol, but dissolves in minute quantity in rectified spirit. Soluble in three parts of water, both hot and cold. Crystallizes in cubes, which are anhydrous. _Impurities of Common Salt._--Table salt often contains large quantities of the chlorides of magnesium and calcium, which, being deliquescent, produce a dampness by absorption of atmospheric moisture: sulphate of soda is also commonly present. The salt may be purified by repeated recrystallization, but it is more simple to prepare the pure compound _directly_, by neutralizing hydrochloric acid with carbonate of soda. Chloride of Silver. (_See_ Silver, Chloride of.) Citric Acid. This acid is found abundantly in lemon-juice and in lime-juice. It occurs in commerce in the form of large crystals, which are soluble in less than their own weight of water at 60°. Commercial citric acid is sometimes mixed with tartaric acid. The adulteration may be discovered by making a concentrated solution of the acid and adding _acetate of potash_; crystals of bitartrate of potash will separate if tartaric acid be present. Citric acid is tribasic. It forms with silver a white insoluble salt, containing 3 atoms of oxide of silver to 1 atom of citric acid. If the citrate of silver be heated in a current of hydrogen gas, a part of the acid is liberated and the salt is reduced to a citrate of _suboxide_ of silver; which is of a red color. The action of white light in reddening citrate of silver is shown by the author to be of a similar nature. Cyanide of Potassium. Symbol, K, C{2}N, or KCy. Atomic weight, 66. This salt is a compound of cyanogen gas with the metal potassium. Cyanogen is not an elementary body, like chlorine or iodine, but consists of carbon and nitrogen united in a peculiar manner. Although a compound substance, it reacts in the manner of an element, and is therefore (like _ammonium_, previously described) an exception to the usual laws of chemistry. Many other bodies of a similar character are known. Ether. Symbol, C{4}H{5}O. Atomic weight, 37. Ether is obtained by distilling a mixture of sulphuric acid and alcohol. If the formula of alcohol (C{4}H{6}O{2}) be compared with that of ether, it will be seen to differ from it in the possession of an additional atom of hydrogen and of oxygen: in the reaction, the sulphuric acid removes these elements in the form of water, and by so doing converts one atom of alcohol into an atom of ether. The term _sulphuric_ applied to the commercial ether has reference only to the manner of its formation.