2. Treat with nitric acid, precipitate with sodium chloride or potassium hydrate, then put with III, above, for reduction.
Washings from Positive Prints.
Precipitate by sodium chloride, potassium
hydrate, or on copper plates, &c., as above, and reduce, as in III.
VI. Cyanide Solution.
1. Dilute with water, precipitate by (1) potassium sulphide, (2) sodium chloride, and reduce as 3.
2. Decant bath into iron kettle, warm, add ferrous sulphate slowly till slight precipitate of oxide. Make alkaline, and add solution of grape sugar till brownish-yellow colour. Allow to settle, syphon off liquid. Wash sediment on filter, and ignite to recover silver.
Note.—Ferrous sulphate forms ferrocyanide, therefore no free alkaline cyanide should be present.
VII. Developer.
1. Vide II. (3, 4, 5, 6), with hypo bath; (1) and (2) not applicable, for sulphide of iron would be formed.