Pure water, that is water which is not mixed with any other substance, is practically unknown. To obtain pure water artificially requires the highest skill of the chemist's art combined with the most delicate technique. Water has been described as the universal solvent. Not only does it dissolve many other liquids, but acts upon most solid bodies producing a greater or less degree of solution, and particularly does it dissolve the gases with which it may come into contact. For this reason water also dissolves the materials of its containers, especially if they be glass, the degree of solution being very marked in some kinds of glass and less marked in others. By reason of its tendency to dissolve matters of this kind water can only be secured in a pure state with the greatest precautions and by preparing it in vessels to which it is inert. Pure water, in the strictest sense of that term, is therefore a chemical curiosity and does not concern us in any other way. The art of preparing pure water is strictly chemical and does not form any part of the present discussion. The distilled water of commerce, when properly prepared is free of living organisms and contains only minute quantities of foreign bodies.