453. Single-solution Mercuric-iodide Intensi fier. Intensification in a single solution of mercuric iodide 2 was described in 1879 by B. J. Edwards. Since mercuric iodide (or bi-iodide of mercury, a very heavy red salt, generally supplied in the form of powder) is insoluble in water, this author proposed dissolving it in a mixture of potassium iodide and of hyposulp hite of soda (in each of which it is separarely soluble). Later, it was found (Lutniere and Seyewetz, 1899) that the use of sulphite instead of hyposulphite permits a slightly more vigorous intensification." The following baths are used— I ii Edwards 1.2tInic're and Seyewelz Potassium iodide . . . 280 gr.
(20 grin.) Soda sulphite, anhydrous _ . goo gr.
(100 grm..) Mercuric iodide . . iSo gr. go gr.
(20 grm.) ( so grin.) Hyposuiphite of soda . . 18o gr. — (2o grin.) Water, to snake _ . 2000. 20 OZ.
(1,000 C.C.) (,,000 C.c.) The first of these solutions contains no oxidizable product, and is more stable than the second. Both can be kept for a long time in the dark (earthenware bottles, or glass bottles covered with black paper). The solutions may be used many times, so long as they are kept away from light when not in actual use.
In these baths the image is intensified progres sively without changing its appearance ; the silver is converted into iodide, whilst at the same time metallic mercury is deposited on The process is stopped when the desired degree of intensification has been obtained ; the negative is then thoroughly washed.
The image intensified by method I is slightly more stable than that intensified by method I I.
In moist air, though more slowly in dry, an image intensified in this way becomes yellowish (probably due to the formation of a complex of mercuric oxide and silver iodide). Absolute stability can be conferred on the image by plac ing the negative, after washing, in an ordinary developing bath, which reduces the silver iodide to metallic silver, or in a x per cent solution of sodium monosulphide (§ 589) which changes the two metals into sulphide without appreciably modifying the densities of the image.
A negative, which for the lack of these pre cautions has suffered this alteration of colour, may at any time be brought back to its original condition after intensification by treating it for a sufficient time with a developer or with the sulphide solution.
The action of this intensifier is not propor tional (Nietz and Huse, 1918) ; the action is greatest with the lower densities, which is a. considerable advantage in most cases of prac tical interest, the shadow parts of a negative requiring more energetic intensification than the image of the high lights. 2 Altogether the mercuric iodode intensifier is one of the most general usefulness. Especially for the purposes of Press photographers, who often need to give a little sparkle to negatives, it has the very great advantage that there is no need for the very long washing for removal of hypo which is required when using most other intensifiers. The possibility of a subsequent change in the negative is often a matter of no importance.
454. Chromium Intensification. About 1880 Eder suggested as a possible method of intensi fication the conversion of the silver of the photo graphic image into silver chloride by means of a solution of bichromate acidified with a little hydrochloric acid, and its re-development in a pyrogallol developer. In this way, in addition to the black image of the reduced silver, there is superimposed on it the brown image which is formed in the gelatine by the oxidation products of the In experimenting with this process, which had been forgotten, C. Welborne Piper and D. J. Carnegie in 1904 found that the intensification was partly due to the deposition of a chromium compound (probably the oxide) in the image, the amount of this deposit being greater if the bleaching solution were only slightly acidified. The active agent in this intensification appeared to them to be a chlorochromate (produced by the action of hydrochloric acid on a bichromate), and they found that the silver chloride thus formed was developable without exposure to light, at any rate if the bleaching bath did not contain a great excess of hydrochloric acid ; the latter, by being partially converted to chlorine, yielded ordinary silver chloride, which was only developable after exposure to light. For darken ing they recommended the use of an amidol developer as lending itself best to the purpose of successive intensifications without causing frilling of the gelatine. The progressive increase of density by repeated intensification has been confirmed by photo-micrographs published in 1916 by W. T. P. Cunningham.