In the collodion film, the image is on the extreme surface, and the particles of silver on the film attract the crystalline silver precipitate which gradually separates out from the depositing solution. In the high lights, where there is a greater conglomeration of silver particles, a larger proportion of metallic crystals are deposited, because the attractive force there is greater. Thus in the intensification of a collodion film the second deposit is in exactly the same proportion as the first, and therefore the delicate gradation of the original image is perfectly preserved after the process of intensification. In the treatment of a gelatine plate with the mercury intensifies the effect is very different, for the image is not only on the extreme surface, but in the denser parts the deposit is situated throughout the thickness of the film. When treated with the mercury solution, the halftones and details in the shadows of the negative are at once increased to double their original density, while only the surface of the deposit in the high lights is attacked; therefore the increase in density of the latter is not more than perhaps 20 or 30 per cent., supposing that the negative under treatment only requires a moderate amount of intensification.

In such a case, the image greatly loses its brilliant contrast or, technically speaking, a general "flatness" in the resulting photographs is noticeable. This tendency of the mercury intensifier to produce "flatness" may, in some few exceptional instances, be turned to good account, as when a negative possesses too much contrast, but requires a slight strengthening of the image. There is still another example where the mercury solution will, if employed, yield results well-nigh perfect - referring to the intensification of very thin images, as are frequently produced with highly sensitive commercial plates. In this latter case, the silver deposit should be subjected to the action of the mercury till the entire image is attacked - often a matter of 7-10 minutes - when, of course, the negative preserves its original delicate gradation after the process of intensification. By mercurial intensification, is meant the process of bleaching the silver deposit with a mercuric salt, and then treating with some compound capable of blackening the image, such as ammonia, sulphuretted hydrogen, soda sulphite, or ferrous oxalate.

The effect of mercuric chloride (the salt usually used) on the silver image is to convert the latter into silver chloride, and, at the same time, to deposit locally mercurous chloride; thus the bleached image consists of silver chloride and mercurous chloride. In the process of blackening, it is generally only the latter that is affected, e. g. ammonia forms the black mercury amido-chlo-ride; while, again, soda sulphite reduces the mercury salt to the metallic state. With sulphuretted hydrogen, or ammonium sulphide, both the chlorides in the image are converted into the corresponding sulphides; also ferrous oxalate reduces the two chlorides to the metallic state.

It is a well-known fact that free acids act as powerful restrainers in preventing the combination of metallic salts with organic substances; as, for example, the use of citric acid in hindering the formation of silver albuminate in sensitised albumenised paper. In the same way, hydrochloric acid prevents the formation of the gelatino-mercury compound. In proof of this assertion, Spiller cites an illustrative experiment. An ordinary gelatine negative was cut in half, one piece was treated with the usual neutral mercuric solution, and the other was immersed in a similar solution, but containing a small proportion of hydrochloric acid; both films were then thoroughly washed in the same dish.

The two plates were next cut up into three, and one piece from each was treated with solutions of ferrous oxalate, ammonium sulphide, and dilute ammonia respectively. On examination, it was found that while all the films that had been immersed in the acid mercury bath presented beautifully brilliant negatives and quite colourless in the shadows, those pieces from the neutral bath were more or less stained. The alkaline sulphide solution developed the most stain, and was of a very non-actinic brown tint; ferrous oxalate yielded a less conspicuous grey deposit, while the ammonia gave the least objectionable result. It was found, however, that the ammonia-treated film turned quite brown on after treatment with the sulphide solution, proving that the ammonia did not remove part of the mercury, but the latter remained to a certain extent in embryo. For the intensification of gelatine films in which a preliminary treatment with mercuric chloride is required, Spiller recommends the following solution: -

Saturated solution of mercury bichloride .. .. 20 oz. Hydrochloric acid (strong) 1/2 dr.

A larger proportion of the restrainer might be added when treating hard films, but for general purposes the above formula is preferable, as the acid is liable to produce frilling.

Although not generally used, and probably unknown to many, the most perfect method for intensifying gelatine films is that in which a mercury compound is used simply as a carrier for silver, but does not actually exist in the final result. The process to which reference is made depends on the reduction of silver cyanide by a mercurous salt; the film is bleached in a solution of mercuric bromide, and, after slight washing, is immersed in a bath of silver cyanide dissolved in potassium cyanide. By the first treatment, the silver image is converted into mercurous and silver bromides, as is illustrated in the following equation: -

Mercuric Silver Mercurous Silver Bromide. Bromide. Bromide.

HgBr2 + Ag = HgBr + AgBr.

In the second reaction the mercurous bromide reduces the silver cyanide to the metallic state, thus : -

Mercurous Potassic Mercuric

Silver Silver Cyanide