Fowler’s solution, copper nitrate.
Soda lye or sodium carbonate, phenolphthaleine.
A sympathetic ink is one that is invisible when written, but which can be made visible by some treatment. Common milk can be used for writing, and exposure to strong heat will scorch and render the dried milk characters visible.
The following inks are developed by exposure to the action of reagents:
I.—Upon writing with a very clear solution of starch on paper that contains but little sizing, 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 hyposulphite of soda (1 in 1,000).
II.—Characters written with a weak solution of the soluble chloride of platinum or iridium become black when the paper is submitted to mercurial vapor. This ink may be used for marking linen, as it is indelible.
III.—Sulphate of copper in very dilute solution will produce an invisible writing, which may be turned light blue by vapors of ammonia.
IV.—Soluble compounds of antimony will become red by hydrogen sulphide vapor.
V.—Soluble compounds of arsenic and of peroxide of tin will become yellow by the same vapor.
VI.—An acid solution of iron chloride is diluted until the writing is invisible when dry. This writing has the 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 {413} by ammonia, and may alternately be made to appear and disappear by these two vapors.