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It is easy to observe in the laboratory the fact of the different behaviour of salt and fresh water towards finely divided substances.  The nature of the insoluble substance is not important.

We place, in a good light, two glass vessels of equal dimensions; the one filled with sea water, the other with fresh water.  Into each we stir the same weight of very finely powdered slate:  just so much as will produce a cloudiness.  In a few hours we find the sea water limpid.  The fresh water is still cloudy, however; and, indeed, may be hardly different in appearance from what it was at starting.  In itself this is a most extraordinary experiment.  We would have anticipated quite the opposite result owing to the greater density of the sea water.

But a still more interesting experiment remains to be carried out.  In the sea water we have many different salts in solution.  Let us see if these salts are equally responsible for the result we have obtained.  For this purpose we measure out quantities of sodium chloride and magnesium chloride in the proportion in which they exist in sea water:  that is about as seven to one.  We add such an equal amount of water to each as represents the dilution of these salts in sea water.  Then finally we stir a little of the finely powdered slate into each.  It will be found that the magnesium chloride, although so much more dilute than the sodium chloride, is considerably more active in clearing out the suspension.  We may now try such marine salts as magnesium sulphate,

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or calcium sulphate against sodium chloride; keeping the marine proportions.  Again we find that the magnesium and calcium salts are the most effective, although so much more dilute than the sodium salt.

There is no visible clue to the explanation of these results.  But we must conclude as most probable that some action is at work in the sea water and in the salt solutions which clumps or flocculates the sediment.  For only by the gathering of the particles together in little aggregates can we explain their rapid fall to the bottom.  It is not a question of viscosity (i.e. of resistance to the motion of the particles), for the salt solutions are rather more viscous than the fresh water.  Still more remarkable is the fact that every dissolved substance will not bring about the result.  Thus if we dissolve sugar in water we find that, if anything, the silt settles more slowly in the sugar solution than in fresh water.

Now there is one effect produced by the solution of such salts as we have dealt with which is not produced by such bodies as sugar.  The water is rendered a conductor of electricity.  Long ago Faraday explained this as due to the presence of free atoms of the dissolved salt in the solution, carrying electric charges.  We now speak of the salt as “ionised.”  That is it is partly split up into ions or free electrified atoms of chlorine, sodium, magnesium, etc., according to the particular salt in solution.  This fact leads us to think that these electrified