PHOTOGRAPHIC CORRESPONDENCE.

Spots on Collodion Pictures, &c.—The principal difficulty I experience in the collodion process is occasioned by the appearance of numberless very minute spots or points over the whole extent of the picture. These occurring on the whites of my pictures (positives) give them a rough, rubbed, appearance and want of density, which I should feel obliged if any of your correspondents can teach me how to overcome.

One of your photographic querists inquires the remedy for his calotype negatives darkening all over before the minor details are brought out. I had for a long time been troubled in the same way, but by diminishing the aperture of my three-inch lens to half an inch, and reducing the strength of my sensitising solution to that given by Dr. Diamond, and, in addition, by developing with gallic acid alone until the picture became tolerably distinct in all its parts, and then applying the gallo-nitrate, I have quite succeeded in obtaining first-rate negatives. It is well to prepare only a small quantity of aceto-nitrate at once, as the acetic acid is of a sufficiently volatile nature to escape from the solution, which is a not unfrequent cause of the general darkening of the picture. It would be well to substitute a more fixed acid for the acetic if this be practicable, as it is in the collodion process, where tartaric is recommended.

H. C. Cowley.

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.