17. Gold chloride serves for forming characters that appear only as long as the paper is exposed to daylight, say for an hour at least.
18. Write with a solution made by dissolving one part of silver nitrate in 1,000 parts of distilled water. When submitted to daylight, the writing appears of a slate color or tawny brown.
Inks Appearing through Reagents.
19. If writing be done with a solution of lead acetate in distilled water, the characters will appear in black upon passing a solution of an alkaline sulphide over the paper.
20. Characters written with a very weak solution of gold chloride will become dark brown upon passing a solution of tin perchloride over them.
21. Characters written with a solution of gallic acid in water will become black through a solution of iron sulphate and brown through the alkalies.
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.