"The first mixture containing only 1/8th of alcohol was quite unfit for photographic purposes, from its being almost impossible, even with the most rapid immersion, to obtain a film of uniform sensitiveness and opacity throughout, the surface generally exhibiting nearly transparent bands, having an iridescent appearance by reflected light. "The second mixture with 1/4th of the alcohol is liable to great uncertainty, for if there be any delay in pouring off the collodion the same appearances are seen as in the first, and like it the surface is very insensitive to light, while if the plate be rapidly plunged in the bath, the collodion film becomes much more opaque than before, and is then very sensitive. "The third proportion of 3 of alcohol to 5 of ether is decidedly the best, giving without the least difficulty a beautifully uniform and highly sensitive film, at the same time perfectly tough and easily removable from the glass if required. A further addition of alcohol, as in the two last collodions, was not attended with any corresponding advantage or increase of sensitiveness; on the contrary, the large proportion of alcohol rendered them less fluid, though with a smaller quantity of gun cotton they would produce very good collodions, capable of giving fine films: the cause of the weakness of the film, observed on adding much of the ordinary alcohol, is the large amount of water it usually contains. "This surprising improvement, caused by the addition of a certain quantity of alcohol, is referable to causes partly chemical, partly mechanical, for, on examining the films, it will be found in the first, and occasionally in the second collodion, that the iodide of silver is formed on the surface, and can be removed entirely by friction without destroying the transparent collodion film below, while in those collodions that contain more than one-fourth of alcohol, the iodide of silver is wholly in the substance, and in this state possesses the utmost sensitiveness. "This difference of condition is owing to the very sparing solubility of ether in water, which in the first case prevents the entrance of the nitrate of silver into the film, consequently the iodide and silver solutions meet on the surface; but on addition of alcohol, its solubility enables the two to interchange places, and thus the iodide of silver is precipitated throughout the substance in a state of the utmost division. This difference is clearly seen under the microscope, the precipitate being clotted in the one case, while in the other the particles are hardly discoverable from their fineness.