Devizes, Wilts.
The Double Iodide Solution.—The great difference in the quantity of iodide of potassium ordered by different persons, to dissolve a given weight of iodide of silver in a given volume of water, has induced me to make some experiments on the subject. I find that using pure nitrate of silver, and perfectly pure iodide of potassium (part of a parcel for which Mr. Arnold, who manufactures iodine on a large scale in this island, got a medal at the Exhibition of 1851), the quantity of iodide of potassium required varies, cæteris 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 ratâ 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 | 97.66 | 146.49 |
Iodide of silver | 68.82 | 137.64 | 206.46 |
Iodide of potassium | 320 | 640 | 960 |
Water up to | 2 oz. | 4 oz. | 6 oz. |
T. L. Mansell, A.B., M.D.
Guernsey.
Mounting Photographs (Vol. ix., p. 282.).—J. L. S. will find the "Indian-rubber glue," which is sold in tin cases, the simplest and cleanest substance for mounting positives; it also possesses the advantage of being free from the attacks of insects.
Seleucus.