CITRATE OF SILVER. 3 Ag. 0, Ci. H 0. This falls as a shin ing white powder on mixing citrate of soda with nitrate of silver, and has been used as the basis of a printing process, for the par ticulars of which the reader is referred to the article on printing. Citrate of silver is insoluble in water, but soluble in ammonia, no doubt forming citrate of ammonia and ammoniacal oxide of silver. When heated to 212° in the presence of hydrogen, part of the oxygen of the oxide of silver in the citrate, is converted into water by uniting with the hydrogen, and citrate of the suboxide of silver instead of citrate of the oxide remains. This has been represented as the change which light also produces on this salt, but it is scarcely a satisfactory explanation. It appears more probable that the citric acid is also at the same time decomposed. If half of the oxygen be removed by light, it must either combine in its nascent state with some hydrogen or carbon in the atmosphere, orit must be entirely set free, or it must oxidize the citric acid or the paper. It is easy to account for its being removed by hydrogen and heat to form water, but when it is separated from the silver by light, it is combined, there can be little doubt, with the organic matter present, and the reduction is due to the joint action of the citric acid and light. Citric acid and the other vegetable acids, all par take in a greater or less degree of the qualities of genic and pyro genic, and are capable of being entirely oxidized into carbonic acid and water. Heat increases this tendency to oxidation by increas ing the affinity of carbon and hydrogen for oxygen or chlorine, and light does the same. Citric ackl will reduce the chloride of gold to the metaffic state, and combine with the chlorine, and when subjected to heat it undergoes changes analogous to the conversion of gallic into pyrogallic. The decomposition take,s places gra dually, and by stages, for citric acid in its oxidation is capable of forming a variety of other organic compounds, and the silver is reduced more and more nearly to the metallic state. The tendency of citrate of silver to be decomposed in this manner is shown by the action of heat upon it. The affinity of the carbon and hydrogen for the oxygen of the silver is increased, by heat, so much that when the salt comes into contact with an incandescent body a kind of explosion takes place, and carbon and metallic silver alone remain.
Many other organic salts of silver comport themselves after the same fashion. It seems natural to conclude that what takes place here, almost instantaneously, is effected by light, according to its usual mode of acting on organic substances, viz., by a series of gradual and progressive substitutions. The gradual approach to t,he metallic state, in prints taken on citrate of silver, is proved by the difference observed in the reaction of nitric acid, ammonia, and hyposulphite of soda upon it in the different stages, by the growing indifference to oxidizing compounds, the greater permanence of the more reduced parts, the greater facility of amalgamating with mercury, and the bronzing or metallic appearance of the strongly sunned portions. The colour also changes from the red which is indicative of organic matter to blue, which is characteristic of metallic silver. No such alteration of properties appears to attend the reduction of pure chloride ; it never bronzes, and appears to be resolved at all stages of the exposure to light into chloride of silver and pure metal. The less citrate of silver prints have been exposed to light the more organic matter will continue in combination with the silver, and the more liable will they be to suffer deterioration by the action of elements for which the organic matter has affinity : the colour is a measure of this liability with the same preparations. When ammonia-nitrate is used with the citrate, the image is bluer and more metallic than with plain nitrate, because alkalis increase the affinity of vegetable acids for oxygen, as is seen in the mixture of ammonia and pyrogallic acid. The decomposed citric salt is more or less acid to the organic compounds of silver which are precipitated by the usual developers, and, therefore, attracts them ; and as the silver which falls from the more powerful developers is more completely reduced than that which citric acid deoxidizes, the developed prints are more permanent than the sun prints.