_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, _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.