solution, we get then a change which we might represent thus:  Copper sulphate, consisting of a combination of copper oxide with sulphuric acid, yields with barium chloride, which is a combination of barium and chlorine, insoluble barium sulphate, a combination of barium oxide with sulphuric acid, and soluble copper chloride, a combination of copper and chlorine.  This is called a double interchange.  Now these are a few illustrations to show you what is meant by chemical decompositions.  One practical lesson, of course, we may draw is this:  We must have a care in dissolving bluestone or copper sulphate, not to attempt it in iron pans, and not to store or put verdigris into iron vessels, or the iron will be acted upon, and to some extent the copper salt will become contaminated with iron.  It will now be clear to you that, as a solvent for bodies usually soluble in water, water that is perfectly pure will be most suitable and not likely to cause any deposition or precipitation through chemical decompositions, for there are no salts or other compounds in pure water to cause such changes.  Such pure water is called soft water.  But the term is only a comparative one, and water that is not quite, but nearly pure—­pure enough for most practical purposes—­is also called soft water.  Now rain is the purest form of natural water, for it is a kind of distilled water.  Water rises in vapour from the ocean as from a still, and the salt and other dissolved matters remain behind.  Meeting cold currents of air, the vapours condense in rain, and fall upon the earth.  After coming in contact with the earth, the subsequent condition of that water entirely depends upon the character, as regards solubility or insolubility, of the substances composing the strata or layers of earth upon which it falls, and through which it sinks.  If it meets with insoluble rocks—­for all rocks are not insoluble—­it remains, of course, pure and soft, and in proportion as the constituents of rock and soil are soluble, in that proportion does the water become hard.  We all know how dangerous acid is in water, causing that water to act on many substances, the iron of iron vessels, the lime in soil or rock, etc., bringing iron and lime respectively into solution.  Now the atmosphere contains carbonic acid, and carbonic acid occurs in the earth, being evolved by decomposing vegetation, etc.  Carbonic acid is also soluble to a certain, though not large extent, in water.  As we shall see, water charged with carbonic acid attacks certain substances insoluble in pure water, and brings them into solution, and thus the water soon becomes hard.  About the close of the last lecture, I said that lime is, to a certain extent, soluble in cold water.  The solution is called lime-water; it might be called a solution of caustic lime.  When carbonic acid gas first comes in contact with such a solution, chalk or carbonate of lime, which is insoluble in water, is formed, and the lime is thus precipitated as carbonate.