_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 re-crystallization, 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. When the Citrate of Silver is 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 colour. 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, KC{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. Properties of Cyanide of Potassium.--These have been sufficiently described at page 44, to which the reader is referred. 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. Properties of Ether.--The properties of Ether have been described to some extent at pages 85 and 195. The following particulars however may be added. It is neither acid nor alkaline to test-paper. Specific gravity, at 60°, about ·720. Boils at 98° Fahrenheit. The vapour is exceedingly dense, and may be seen passing off from the liquid and falling to the ground: hence the danger of pouring Ether from one bottle to another if a flame be near at hand.