194. The statement that silver chloride dissolves in a solution of calcium hyposulphite is not strictly accurate. It is decomposed, exchanging constituents with the calcium salt, forming calcium chloride and silver hyposulphite. The latter combines with another portion of calcium hyposulphite, and forms a double salt, silver calcium hyposulphite, which is very soluble in water.

If calcium pentasulphide is added to such a solution in equivalent proportion, one-fifth of its sulphur combines with silver to form silver sulphide, which is precipitated, together with the remaining four-fifths of the sulphur. The oxygen, and so-called hyposulphurous acid, which were combined with the silver, combine with the calcium of the decomposed precipitant. Thus the portion of the solvent which was decomposed by silver chloride is reproduced.

From this it follows that, for every atom of silver extracted from the ore, and precipitated by polysul-phide, the solvent gains a molecule of calcium chloride, without however having ultimately lost any hyposulphite, and this is the reason why the density of the solution ceases to be an index of its solvent power. So far there is neither a loss nor a gain of calcium hyposulphite in the dissolving solution, but, as stated elsewhere, the precipitant always contains a quantity of that salt in watery solution, which being added to the solvent, increases its volume without impairing its strength, unless the precipitant is too much diluted.

In practice the increase or diminution in strength and volume of the solvent depends on the care exercised in saving as much of it, with as little addition of wash water, as possible, and in having the precipitant sufficiently concentrated. Kustel gives 6° Beaume, as a minimum density for the latter.