_Illustration from the Non-metallic Elements._--If a stream of Chlorine gas be passed into a solution containing the same salt as before mentioned, viz. the Iodide of Potassium, the result is to liberate a certain portion of Iodine, which dissolves in the liquid, and tinges it of a brown colour. The element Chlorine, possessing a degree of chemical energy superior to that of Iodine, prevails over it, and removes the Potassium with which the Iodine was previously combined. Chlorine + Iodide of Potassium = Iodine + Chloride of Potassium. _The same Law illustrated by the Metals._--A strip of Iron dipped in solution of Nitrate of Silver becomes immediately coated with metallic Silver; but a piece of Silver-foil may be left for any length of time in Sulphate of Iron without undergoing change: the difference depends upon the fact, that metallic Iron has a greater attraction for Oxygen than Silver, and hence it displaces it from its solution. Iron + Nitrate of Silver = Silver + Nitrate of Iron. _Illustrations amongst Binary Compounds._--If a few drops of solution of Potash be added to solution of Nitrate of Silver, a brown deposit is formed, which is the Oxide of Silver, sparingly soluble in water. That is to say, as a stronger metal displaces _metallic Silver_, so does an oxide of the same metal displace _Oxide of Silver_. Therefore bases like the alkalies, alkaline earths, etc. cannot exist in a free state in solutions of the salts of weaker bases,--a liquid containing Nitrate of Silver could not also contain free Potash or Ammonia. In the list given at page 306, the metallic elements are arranged principally in the order of their chemical affinities; those of Potassium, Sodium, Barium, etc. being the most marked. As the alkalies displace the weaker bases from their combination with acids, so the strong _acids_ displace weak acids from their combination with bases. Thus, as Oxide of Potassium + Acetate of Silver = Oxide of Silver + Acetate of Potash; So Nitric Acid + Acetate of Silver = Acetic Acid + Nitrate of Silver. In the list of acids. Sulphuric Acid is usually placed first as being the strongest, and Carbonic Acid, which is a gaseous substance, last. The vegetable acids, such as Acetic, Tartaric, etc., are _intermediate_, being weaker than the mineral acids, but stronger than Carbonic, or Hydrocyanic Acid. _The order of decompositions affected by the insolubility or the volatility of the products which may be formed._--It might be inferred from remarks already made, that on mixing saline solutions, a gradual interchange of elements would take place, until the strongest acids were associated with the strongest bases, and _vice versâ_. There are many causes however which interfere to prevent this; one of which is _volatility_.---