(Take of nitrate of silver, 148.75 grains; distilled water, one pint. Dissolve, and keep in an opaque stoppered bottle. The quantity of this solution which fills the volumetric tube to 0, includes seventeen grains of nitrate of silver, or 1/10 of an equivalent of the salt in grains.) The volumetric solution of nitrate of silver is made of such a strength that one hundred measures contain 17 grains (or 1/10 of an equivalent) of nitrate of silver (AgO, No5 = 170). It is used for determining the strength of hydrocyanic acid. When nitrate of silver is added to a solution of hydrocyanic acid, to which an excess of soda has been added, it gives rise to the formation of a double salt, consisting of one equivalent of cyanide of sodium and one equivalent of cyanide of silver (NaCy, AgCy) which is precipitated at first, but re-dissolved on agitation. When all the hydrocyanic acid is withdrawn in the formation of this double cyanide, nitrate of silver gives rise to a precipitate no longer soluble; the appearance of this permanent precipitate is an indication that all the cyanogen is exhausted. The changes may be exhibited in the formula (AgO, No5 + 2 Na Cy = NaO,No5 + NaCy, AgCy). Hence it will be seen that each equivalent of nitrate of silver represents two of hydrocyanic acid. So that 17 grains (or 1/10 of an equivalent) of nitrate of silver will correspond to 5.4 (2.7 x 2) grains, or 2/10 of an equivalent in grains, of absolute hydrocyanic acid.