INVISIBLE WRITING.

Press, Volume LVIII, Issue 17611, 14 November 1922, Page 4

INVISIBLE WRITING.

There are certain liquids which, when used for writing on pjaper, yield characters ihat are invisible when dry, hut are immediately revealed by the application of heat or of an appropriate chemical agent; Such writing-fluids are known as invisible or> sympathetic inks. The simplest "developer" of hidden writing is heat. Messages written in milk, lemon juice, onion juice, or welldiluted oil of vitriol are invisible when dry, but reveal themselves when the paper is he'd jin front of the fire. But the most striking results of development by heat are obtained with salts of cobalt and of nickel. The property that certain, compounds of cobalt possess of .turning blue when heated is utilised to give a very pretty effect. A letter written with a solution of chloride of cobalt; in-water is visible only as faint pink lines. On pink paper the "writing is qiiite.invisible, but when heated it appears of a bsautiful blue colour, which gradually 'disappears on exposure to cold air or by breathing on it. Writing done with a mixed' solution of chloride of. cobalt and chloride of nickel gives a bright green on heating, the shade.varying according to the relative proportions of the two salte used. The writing fades away on cooling. A so'ution of equal parts of sulphate of copper and salammoniao in water, writes J. Gieig in the "Boy's Ow.n Paper," gives an invisible writing, which appears in yellow, characters when heat is applied. Writing done with sulphate of copper dissolved alone comes forth of a lino blue when held over the mouth of an ammonia bottle. A solution of sulphnto of iron, when iised for writing, may he developed black by infusion of oak-galls or by a solution of tannin; by applying instead a solution of yellow prussinte of pbtash it is brought out in blue. -The solution of prussiate.of potash, when used for writing, can,\>f course, be developed in bine by reversing the prce:ss, but by using solution of sulphate of copper to develop the writing appears red. Writing done with boiled starch has to be developed by moistening it with water to which a few drops of tincture of iodine has been added. It then stands out of a deep indigo colony Photographers whose pictorial efforts do not end with the pressing of a button, may exchange greetings secretly by using a odourless solution of "pyro." as the medium.

The most magnificent tomb in the world is the Taj Mahal, in Agia, Hindustan. It was erected "by Shah Jehan to the memory of, his favourite Queen. It is octagonal in form, of pure white marble ? inlaid with . jasper, cornelian, turquoise, agate, amethysts, ' and sapphires. The work took 22,000 men 20 years to complete, and though there were free gifts and the labour was free, the cost was £3,500,000.