Causes of Failure. 295 



pared with the degree to which the collodion has been iodized, 

 tracks will be left in the path followed by the fluid during the 

 act of immersion, and the sensitization will be unequal. When 

 a silver bath becomes weak by use, the iodo-nitrate of silver 

 crystallizes, and the crystals settle on the collodion, adhering 

 strongly to it, and causing it, after fixing, to be full of small 

 holes. Since this excess of iodo-nitrate cannot be removed by 

 filtration, it must be decomposed into iodide and nitrate by 

 greatly weakening the bath with water. The iodide then pre- 

 cipitates and is retained by a filter ; and the strength of the 

 bath is restored by adding nitrate of silver in proportion to 

 the quantity of water used in weakening. The bath may, 

 however, be kept from this inconvenient state by adding, 

 daily, as much nitrate as is removed by the plates. 



If an old, or partially dried filter is used with the silver 

 bath, the molecules of silver deposited on it, on account of the 

 decomposition caused by the organic matter, will take oxygen 

 from some of the nitric acid. The bath will then be slightly 

 yellow, and will contain a soluble nitrate and an insoluble basic 

 nitrate ; and only grey, clouded, and indistinct pictures will 

 be produced. A few drops of an alcoholic solution of iodine 

 will remove the excess of silver which is in the subsalt, and 

 bring the bath to a proper condition. If, when the plate is 

 lifted from the bath, the collodion presents a greasy appearance, 

 there is too much alcohol in the bath ; it is derived from the 

 collodion, the ether of which passes off more readily by evapora- 

 tion. The alcohol is got rid of, by keeping the fluid for a few 

 moments at nearly the boiling point. Greasiness would lead 

 to want of uniformity in development. 



If after a collodionized plate, which is to be used in the 

 humid way, has been taken from the silver bath, it is carefully 

 washed, before exposure, all kinds of stains and imperfections 

 will be prevented, whatever the state of the bath ; but, in de- 

 veloping, a quantity of nitrate of silver, sufficient to replace 

 what has been removed by the washing, must be added. The 

 sensibility is somewhat diminished, and the development is 

 more tedious, but the plate may be left for many hours in the 

 water previous to use, which, in some cases, is an advantage — 

 as, for instance, in taking a photograph of the moon, when it 

 may be necessary to wait until clouds have passed. 



The use of gutta-percha for the silver bath is found to 

 produce cloudiness and streaks in the collodion proofs, on 

 account of the large quantity of tannin which it contains. It 

 is supposed also that the gutta-percha itself, though it resists 

 strong acids, is attacked by some of the materials used. The 

 presence in the negative bath of a very small amount of a 

 reducing agent produces serious effects on the film. A gutta- 



