22. Upon writing on paper that contains but little sizing with a very clear solution of starch, and submitting the dry characters to the vapor of iodine, or passing over them a weak solution of potassium iodide, the writing becomes blue, and disappears under the action of a solution of sodium hyposulphite in the proportions of 1 to 1,000. 23. Characters written with a 10 per cent. solution of nitrate of protoxide of mercury become black when the paper is moistened with liquid ammonia, and gray through heat. 24. Characters written with a weak solution of the soluble platinum or iridium chloride become black when the paper is submitted to mercurial vapor. This ink may be used for marking linen. It is indelible. 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. 34. Write with a solution of paraffin in benzol. When the solvent has evaporated, the paraffin is invisible, but becomes visible on being dusted with lampblack or powdered graphite, or smoking over a candle flame.