Lastly, those methods which give great intensification are specially useful in saving neg ives which have only a faint trace of image and which cannot be replaced underbetter conditions.
The use of mercury intensifiers is to be avoided for very small negatives which require consider able enlargement, because the graininess is often very much increased. A negative or positive which is to be preserved should not be treated with a salt of mercury unless subsequent treat ment is given, which reduces the mercury to the metallic state, or to the state of sulphide; in any other case the image is certain to be destroyed after a more or less prolonged period.
447. Mercury Intensification in Two Succes sive Baths. When a negative, in which the image is almost wholly metallic silver, is treated in a solution of mercuric the silver is converted into a double mercurous-silver chloride,' a complex white salt having proper ties which are slightly different from those of a simple mixture of silver chloride and mer curous chloride or calomel. In this way the silver adds to itself about double its weight of mercury (2oo parts of mercury to 108 parts of silver). At this stage of the operation there is exact proportionality between the original density and the new density, the latter being considerably less than the original density.
As a rule, a solution is used containing about 260 gr. of mercuric chloride and 50 minims of hydrochloric acid or nitric acid in 20 oz. (30 grin, mercuric chloride and 5 c.c. acid in i,000 c.c.). Such a solution hardly deteriorates at all on keeping ; used solutions may be kept for further use until the active substance is exhausted.
During the first few moments in the bath the image darkens, and by transmitted light shows a violet tint ; it then gradually becomes white. For proportional intensification the treatment is continued until the image, when viewed from the back, is seen to be completely whitened. In the case of a hard negative in which it is desired to intensify the parts of least density, the action of the mercuric chloride may be interrupted before the heavier densities are completely bleached.
After it has been removed from the bleaching bath the negative must be washed in several changes of water before being Similar careful washing must also be carried out after darkening by any of the processes now to be described.
448. Darkening with Ammonia. The image, bleached and washed as described above, turns brown and then black almost instantly when it is placed in a very dilute solution of ammonia (about 4 dr. of strong ammonia in 20 oz. of water, i.e. 30 c.c. per litre). The greater part of the silver chloride in the image is dissolved by the ammonia and takes with it an appreciable amount of the mercury, whilst the residual mercuric salt is converted into a black substance, which is very opaque but not very stable.' If the image fades after such intensification it cannot be regenerated.
The ammonia solution employed for blacken ing each negative must be thrown away after use.
As we have already noted, this method of intensification, which is not suitable for nega tives in tone nor for those to be kept, re duces the clear tones (or at any rate does not intensify them) whilst it strengthens the heavy densities. 2 Transfer from a very acid bath to an alkaline bath often causes numerous markings and some times reticulation of the gelatine ; it has been recommended, in order to diminish the liability to these defects, to avoid all rubbing of the film during and after its treatment with ammonia, and to add alum (about 3 per cent) to the solution of mercuric chloride.
449. Darkening with Sulphite of Soda. A con siderable improvement in the mercury intensi fication process is made by using sulphite of soda (C. Scolik, 1884) instead of ammonia for darkening the image.
The image darkens almost instantly in a solution of sulphite of soda ; half of the silver is found in the darkened image, together with a quarter of the mercury which was associated with it, both metals being for the most part' reduced to the metallic condition (Chapman Jones, 1894), whilst the other half of the silver and three-quarters of the mercury go into solu tion as complex sulphites. Secondary reactions also slowly occur between the metals in the image and in the solution, if the contact is main tained ; there is a deposition of silver, which is to some extent added to the image, and to some extent replaces part of the mercury.