SYMPATIIETIC nut SECRET Isla. These are fluids which produce colorless, writing upon paper: but. when the latter is subsequently ex posed to heat or proper chemical action, t Ile writing beeomes colored and clearly readable. Among the favorite secret inks may be mentioned sollitions of lead acetate, whose invisible marks on paper turn black on exposure to sulphureted hydrogen, owing to the formation of sulphide of lead. Cobalt nitrate solution furnishes another favorite secret ink, its marks on paper turning blue on treatment with oxalic acid. solution of either cobalt chloride or the nitro-chloride Will yield characters which are green when heated. but which disappear on cooling: also a weak solution of nickel and cobalt chlorides will produce green tracings.
INK-Puwiwits. These are nothing hut the common constituents of ink in a dry state. Black ink-powder may be made by mixing 4 parts of powdered nutgalls. 2 parts of ferrous sulphate, and one part of gum arable. The dry aniline colors may be similarly used.
Ally ink used in printing is essentially a pigment of the required color, mixed with an oil or varnish. John Underwood in IS57 before the Society of :\rts described the prop erties essential to a good printing ink as fol lows: It must distribute freely and easily- and work sharp and clean; it must not have too much tenacity for the type, but conic off freely and adhere to the paper; it, must dry almost immediately on the paper. and not dry :it all on
the type or rollers; it should tie practically proof against the effects of time and chemical reagents. and never eliange color. The pigment used flu Ow best black ink is lampblack. and the vehicle is usually linseed oil. Mineral pigments are generally used to produce the different colored printing•inks. Details concerning the manufac ture of printing-inks may be found in any of the larger handbooks on printing. In lithography. the writing-ink for drawing on the stone may consist of equal parts of tallow, wax, soap, and shellac, mixed up with a sufficient quantity of Paris black. The printing-ink used to take the impression of engraved plates. with a view to transferenee to the stone. may consist of equal parts of tallow, wax, soap. shellac, and pitch.
Consult: Spon, lVorl:shop Receipts (London.
1883) ; Lehner, Tintenfabrikation (Vienna, 1885); Schluttig and Neumann, Die Ei&enyallus tintcn; Grundlayen zu ihrer Beurteilung (Dres den. 1890) ; Naquet, Legal Chemistry, trans. by Battershall (New York, 1876) ; Frazer, Biblio ties, or the Study of Documents (Philadelphia, 1901).