Iodide Of Silver

IODIDE OF SILVER--Perfectly pure, undergoes very little change under the influence of light or heat; but if a very slight excess of the nitrate of silver be added it becomes infinitely more senitive than the chloride

The spectrum impressed upon paper prepared with a weak solution of the hydriodate of potash presents some very remarkable peculiarities. The maximum of intensity is found at the edge of the most refrangible violet rays, or a little beyond it, varying slightly according to the kind of paper used, and the quantity of free nitrate of silver present. The action commences at a point nearly coincident with the mean red of the luminous spectrum, where it gives a dull ash or lead color, while the most refrangible rays impress a ruddy snuff-brown, the change of tint coming on rather suddenly about the end of the blue or beginning of the violet rays of the luminous spectrum. Beyond the extreme violet rays, the action rapidly diminishes, but the darkening produced by these invisible rays, extends a very small space beyond the point at which they cease to act on the chloride of silver.

In its photographic application, it is, alone, of very little use; but in combination with other reagents it becomes exquisitely sensitive. With gallic acid and the ferrocyanate of potash it forms two of the most sensitive photographic solutions with which we are acquainted. These are used in the calotype process.