_Caution!_--As the globules of potassium after conversion into potash, when thrown on ice or water burst, strewing small particles of caustic hot potash in every direction, the greatest care should be taken to keep at a sufficient distance whilst performing the above experiment. Saltpeter, or niter, is a compound of this metal (or rather its oxide) with nitric acid. It is one of the ingredients of gunpowder, and has the property of quickening the combustion of all combustible bodies. Mix some chlorate of potash with lump sugar, both being powdered, and drop on the mixture a little strong sulphuric acid, and it will instantly burst into flame. This experiment also requires caution. Want of space precludes us from considering the individual metals and their compounds in detail; it must suffice to describe some experiments showing some of their properties. The different affinities of the metals for oxygen may be exhibited in various ways. The silver or zinc tree has already been described. Experiments. 1. Into a solution of nitrate of silver in distilled water immerse a clean plate or slip of copper. The solution, which was colorless, will soon begin to assume a greenish tint, and the piece of copper will be covered with a coating of a light gray color, which is the silver formerly united to the nitric acid, which has been displaced by the greater affinity or _liking_ of the oxygen and acid for the copper. 2. When the copper is no longer coated, but remains clean and bright when immersed in the fluid, all the silver has been deposited, and the glass now contains a solution of _copper_. Place a piece of clean iron in the solution, and it will almost instantly be coated with a film of _copper_, and this will continue until the whole of that metal is removed, and its place filled by an equivalent quantity of _iron_, so that the nitrate of _iron_ is found in the liquid. The oxygen and nitric acid remain unaltered in quantity or quality during these changes, being merely transferred from one metal to another. A piece of zinc will displace the iron in like manner, leaving a solution of nitrate of zinc. Nearly all the colors used in the arts are produced by metals and their combinations; indeed, one is named _chromium_, from a Greek word signifying color, on account of the beautiful tints obtained from its various combinations with oxygen and the other metals. All the various tints of green, orange, yellow, and red, are obtained from this metal. Solutions of most of the metallic salts give precipitates with solutions of alkalies and their salts, as well as with many other substances, such as what are usually called prussiate of potash, hydro-sulphuret of ammonia, etc.; and the colors differ according to the metal employed, and so small a quantity is required to produce the color that the solutions before mixing may be nearly colorless. Experiments.