25. C. Widemann communicates a new method of making an invisible ink to Die Natur. To make the writing or the drawing appear which has been made upon paper with the ink, it is sufficient to dip it into water. On drying, the traces disappear again, and reappear by each succeeding immersion. The ink is made by intimately mixing linseed oil, 1 part; water of ammonia, 20 parts; water, 100 parts. The mixture must be agitated each time before the pen is dipped into it, as a little of the oil may separate and float on top, which would, of course, leave an oily stain upon the paper.
26. Write with a solution of potassium ferro-cyanide, develop by pressing over the dry, invisible characters a piece of blotting paper moistened with a solution of copper sulphate or of iron sulphate.
27. Write with pure dilute tincture of iron; develop with a blotter moistened with strong tea.
28. Writing with potassium iodide and starch becomes blue by the least trace of acid vapors in the atmosphere or by the presence of ozone. To make it, boil starch, and add a small quantity of potassium iodide in solution.
29. Copper sulphate in very dilute solution will produce an invisible writing, which will turn light blue by vapors of ammonia.
30. Soluble compounds of antimony will become red by hydrogen sulphide vapor.
31. Soluble compounds of arsenic and of tin peroxide will become yellow by the same vapor.
32. An acid solution of iron chloride is diluted till the writing is invisible when dry. This writing has the remarkable property of becoming red by sulphocyanide vapors (arising from the action of sulphuric acid on potassium sulphocyanide in a long necked flask), and it disappears by ammonia, and may alternately be made to appear and disappear by these two vapors.
33. Writing executed with rice water is visible when dry, but the characters become blue by the application of iodine. This ink was much employed during the Indian mutiny.