xhibition of 1851), the quantity of iodide of potassium
required varies, _caeteris paribus_, to the extent of 15 per cent., with the
quantity of water added to the iodide of silver before adding the iodide of
potassium; the minimum required being when the two salts act on each other
in as dry a form as possible. Take the precipitate of iodide of silver, got
by decomposing 100 grains of nitrate of silver with 97.66 grains of iodide
of potassium; drain off the last water completely, so that the precipitate
occupies not more than five or six drachms by measure; throw on it 640
grains of iodide of potassium; rapid solution ensues; when perfectly clear,
add water up to four ounces: the solution remains unclouded. But if two or
three ounces of water had been first poured on the iodide of silver, 680
grains, as I stated in my former paper, would have been required, and
perhaps 734. The _rationale_ is, I suppose, that in a concentrated form the
salts act on each other with greater energy, and a smaller quantity of the
solvent is required than if it is diluted. Many analogous cases occur in
chemistry. I hope this little experiment will be useful to others, as a
saving of 15 per cent. on the iodide of potassium is gained. As a large
body of precipitated iodide of silver can be more completely drained than a
smaller quantity, in practice it will be found that small precipitates
require a few grains more than I have stated: thus, throw on the
precipitate of iodide of silver (got from 150 grains of nitrate), drained
dry, 960 grains of iodide of potassium; solution rapidly ensues, which,
being made up to six ounces, the whole remains perfectly clear; whereas the
iodide of silver thrown down from 50 grains of nitrate, similarly treated
with 320 grains of iodide of potassium, and made up to two ounces (the
proportional quantities), will probably require 10 or 15 grains more of
iodide to effect perfect solution, the reason being that it contained a
greater quantity of water _pro rata_ than the first.

The following table, showing the exact quantities of iodide of potassium
required to decompose 50, 100, and 150 grains of nitrate of silver, the
resulting weight of iodide of silver, and the weight of iodide of potassium
to make a clear solution up to 2, 4, and 6 ounces, will often be found
useful:

_Grs._ _Grs._ _Grs._
Nitrate of silver 50 100 150
Iodide of potassium 48.83