The terms " invisible " and "sympathetic" are applied to any writing fluid which leaves no visible trace of the writing on the paper, until developed by the application of heat or chemical reagents. They have been suggested (somewhat impractically it must be owned) for use on post-cards. They are principally as follows: - (a) Solution of sugar of lead in pure water leaves no trace of writing when dry; the written characters held over a jet of sulphuretted hydrogen are developed of an intense black colour. (6) Nitrate of the deutoxide of copper in weak solution gives an invisible writing, which becomes red by heating, (c) Chloride of copper in very dilute solution is invisible till heated. To make it, dissolve equal parts of blue vitriol and sal-ammoniac in water, (d) Nitrate of nickel and chloride of nickel in weak solution form an invisible ink, which becomes green by heating, when the salt contains traces of cobalt, which usually is the case; when pure, it be-comes yellow, (e) Chloride of cobalt in properly-diluted solution (25 gr. to the oz.) will produce a pink writing, which will disappear when thoroughly. dry, become green when heated, disappear when cold, and pink again when damp.

When often or strongly heated, it will at last become brown-red. (/) When the solution of acetate of protoxide of cobalt contains nickel or iron, the writing made by it will become green when heated; when it is pure and free from these metals, it becomes blue, (g) Bromide of copper gives a perfectly invisible writing, which appears very promptly by a slight heating, and disappears perfectly by cooling. To prepare it, take 1 part bromide of potassium, 1 part blue vitriol, 8 parts water. It is better also to discolour the blue vitriol with 1 part alcohol. (A) 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, (i) Writing with iodide of potash and starch becomes blue by the least trace of acid vapours in the atmosphere^ or by the presence of ozone. To make it, boil starch, and add a small quantity of iodide of potassium in solution, (j) Sulphate of copper in very dilute solution will produce an invisible writing, which will turn light-blue by vapours of ammonia, (k) Soluble compounds of antimony will become red by sulphide of hydrogen vapour. (l) Soluble compounds of arsenic and of peroxide of tin will become yellow by the same vapour, (m) An acid solution of chloride of iron is diluted till the writing is invisible when dry.

This writing has the remarkable property of becoming red by sulpho-cyanide vapours (arising from the action of sulphuric acid on sulpho-cyanide of potassium in a long-necked flask), and it disappears by ammonia, and may alternately be made to appear and disappear by these two vapours, (n) Writing executed with rice-water is invisible when dry, but tha characters become blue by the application of iodine. This ink was much employed during the Indian Mutiny, (o) Characters written with an aqueous solution of iodide of starch disappear in about 4 weeks. (p) Dissolve 1 fl. oz. common oil of vitriol (sulphuric acid) in 1 pint soft water; stir well, and allow to cool. Write with a clean steel pen; when dry, the writing is invisible; held to the tire, it becomes indelibly black. (q).. Writing executed with a clean quill peri dipped in onion or turnip juice is invisible when dry; when the paper is heated, the characters assume a brown colour, (r) Milk makes a good invisible ink, and butter-milk answers the purpose better. It will not show if written with a clean new pen, and ironing with a hot flat-iron is the best way of showing it up.

All invisible inks will show on glazed paper; therefore unglazed paper should be used. (s) Boil nutgalls in aquavitse; put some Roman vitriol and sal-ammoniac to it, and when cold dissolve a little gum-arabic, and it will, when written with, vanish in 24 hours, (t) Burn flax so that it may be rather mouldered than burned to ashes, then grind it with a muller on a stone, putting a little aquavitse to it, then mix it with a little weak gum-water, and what you write, though it seem fair, may be rubbed or washed out. (u) Widemann communicates a new method of making an invisible ink to Die Natwr. To make the writing or drawing which has been made upon paper with the ink appear, 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; water, 100. 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.