B. WITH COAL-TAR COLOURS.—The colouring matters derived from coal-tar may all be employed for writing purposes. These inks possess bright colours, do not precipitate their colour, and dry quickly. When dried up or thickened, they can be put right by simple dilution with water. On the other hand, they are readily destroyed by chemical reagents. They must not be used with pens which have been employed in writing with other inks. They do not require any addition of gum ; but if desired, 1 part dextrine may be added to every 100 parts ink. Almost all tints may be produced by mixtures, in varying proportions, of the following principal colours Bed : (a) 1 part magenta in 150-200 parts hot water. (b) Dissolve 25 parts (by weight) safranine in 500 parts warm glycerine ; then stir in carefully 500 parts alcohol, and 500 parts acetic acid ; dilute in 9000 parts water, containing a little gum arable in solution.
Blue: 1 part soluble blue (night blue) iu 200-250 parts hot water.
Violet : 1 part violet-blue in 200 parts hot water.
Green : 1 part iodine-green in 200 parts hot water. Gives a bluish-green writing ; for a lighter tint, add a little picric acid.
Yellow : 1 part picric acid in 120-140 parts water. This is not very successful.
Invisible or Sympathetic Ink.—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. (b) 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 becomes yellow. (e) Chloride of cobalt in properly diluted solution 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. (f) 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. (1) A drawing or writing made with a strong solution of acetate of lead becomes dark-brown by exposure to sulphide of hydrogen gas. (1) 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. (1) Soluble compounds of arsenic and of peroxide of tin will become yellow by the same vapour. (n) An acid solution of chloride of iron is diluted till the writiog is invisible when dry. This writing has the remarkable property of becoming red by sulpho-cyanide vapours, 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 the characters become blue by the application of iodine. This ink was much employed during the Indian Mutiny. (c) 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 fire, it becomes indelibly black. (q) Writing executed with a clean quill pen dipped in onion or turnip juice is invisible when dry ; when the paper is heated, the characters assume a brown colour.