Thus, _Cyanide of Potassium_ is a true salt, although highly poisonous; Nitrate of Silver is a salt; the green Sulphate of Iron is a salt; so also is Chalk or Carbonate of Lime, which has neither taste, colour, nor smell. _On the "Hydracid" class of Salts._--The distinction between Oxyacids and Hydracids has already been pointed out (p. 309), the latter having been shown to consist of Hydrogen united with elements analogous in their reactions to Chlorine, Iodine, Bromine, etc. In a salt formed by an Oxygen Acid, both the basic and acid elements appear. Thus the common Nitre, which is a Nitrate of Potash, is found by analysis to contain Oxide of Potassium as a base, in a state of combination with Nitric Acid. But if a salt be formed by neutralizing an alkali with a _Hydrogen Acid_, the product in that case does not contain all the elements. This is seen from the following example:-- Hydrochloric Acid + Soda = Chloride of Sodium + Water; or, stated more at length,-- (Chlorine Hydrogen) + (Oxygen Sodium) = (Chlorine Sodium) + (Oxygen Hydrogen). Observe that the Hydrogen and Oxygen, being present in the correct proportions, unite to form Water, which is an Oxide of Hydrogen. This water passes off when the solution is evaporated, and leaves the dry crystals of salt. On the other hand, with the Oxyacid Salts, the elementary Hydrogen being absent, no water is formed, and the Oxygen remains. It must therefore be borne in mind that salts like the Chlorides, Bromides, Iodides, etc. contain only _two_ elements; but that in the Oxyacid Salts, such as Sulphates, Nitrates, Acetates, _three_ are present. Thus, Nitrate of Silver consists of Nitrogen, Oxygen, and Silver, but Chloride of Silver contains simply Chlorine and metallic Silver united, without Oxygen. The Hydracid salts however, when decomposed, yield products similar to the Oxyacid salts. For instance, if Iodide of Potassium be dissolved in water, and dilute Sulphuric Acid added, this acid, being powerful in its chemical affinities, tends to appropriate to itself the alkali; but it does not remove _Potassium_ and liberate _Iodine_, but takes the _Oxide_ of Potassium and sets free _Hydriodic Acid_, In other words, as an atom of water is produced during the _formation_ of a Hydracid Salt, so is an atom destroyed and made to yield up its elements in the _decomposition_ of a Hydracid Salt. The reaction of dilute Sulphuric Acid upon Iodide of Potassium may be stated thus:-- Sulphuric Acid _plus_ (Iodine Potassium) _plus_ (Hydrogen Oxygen) _equals_ (Sulphuric Acid, Oxygen Potassium) or Sulphate of Potash, _and_ (Hydrogen Iodine) or Hydriodic Acid. THE NATURE OF CHEMICAL AFFINITY FURTHER ILLUSTRATED.