EXISTENCE OF SUB-SALTS DOUBTFUL.
The evidence for and against the idea that the latent image consists of silver may now be examined. According to Wetzlar (Schweig's "Jahrbuch," 1828), he obtained small quantities of silver sub-chloride by treating solutions of ferric and cupric chloride with silver leaf. In 1839 \Vdhler passed a current of hydrogen over silver citrate heated to 100°C. On analysing the residue, a compound having the formula Ag,0 was said to be present. This Ag.,0 would be silver suboxide and would corre spond to the sub-chloride Ag,,C1. Von Llibra (" Journal fur Prak. Chem." [2] treated Wohler's suboxide with strong hydrochloric acid for some time, and isolated a body having the formula If this is the formula for the sub-chloride it contains a greater per centage of silver than that represented by Mmy chemists have repeated the work of Milder mid Von Bibra, and have come to the conclusion, that the existence of these salts is extremely doubtful. Gfintz says that silver sub-chloride, is obtained by treat ing sub-fluoride, Ag,F, with strong hydro chloric acid. He obtained the sub-fluoride by passing a powerful current of electricity through a concentrated solution of silver fluoride, using silver electrodes. Up to the present, the formula of this sub-fluoride has not been es tablished. Otto Vogel ("Phot. Mitt." [36] 334) attempted to isolate these silver sub-haloids by allowing cuprous chloride, bromide, and iodide to act upon a solution of silver nitrate. On a cupric salt silver nitrate acts as under :— CuBr, + 2AgNO, = 2AgBr + With a cnprous salt the reaction is expressed by Vogel as, + = + Cl, The three sub-haloids were stable in the air and only very slightly acted upon by the light. The analytical results agreed with the Ag,C1„ Ag, Br,, and Agj„. Vogel finds that mercury does not extract silver from these haloids. Under the microscope complete
geneity is observed. Nitric acid attacks the compounds, dissolving silver and leaving the ordinary haloids, AgBr, and AgI. Vogel explains the fact,that silver chloride and brom i de are not decomposed by nitric acid when posed to light (which should be the case if they are converted into sub-haloids), by assuming that the sub-haloid at once bines with the ordinary haloid of silver, to form a stable substance. Waterhouse(" Photo. Jour.," 19001 and Eniszl ("Zeits. Anorg. Chem.") peated Vogel's experiments, and came to the clusion that sub-haloids are not produced in this manner. From their investigations it appears that Vogel's compounds consist of intimate mixtures of finely divided silver and unaltered haloid. The latent image cannot consist of duced silver and unaltered haloid, as some have suggested, since silver chloride darkens under nitric acid, and on examining the acid no appreciable amount of silver is found. If the metal were produced by the light action. it would be removed as fast as formed, and the acid would contain silver nitrate.
TuE " PHOTO SALTS " OF CA.REY LEA.
By treating ammoniacal solutions of silver chloride with ferrous sulphate, washing the resulting precipitate, and then treating with hydrochloric acid, various coloured compounds are obtained, containing less halogen than the original chloride. These substances were termed " photo salts " by their discoverer, Carey Lea ( " Amer. Jour. Science," 1887 ), as he considered them identical with the compounds produced by the action of light on the silver haloids. Their method of formation suggests the possibility of sub-salts existing in admix ture with the ordinary haloid. So far no chemical formula can be assigned with cer tainty to these " photo salts."