On many occasions it is of importance to employ an ink indestructible by any process, that will not equally destroy the material on which it is applied. Mr. Close has recommended for this purpose, 25 grains of copal in powder dissolved in 200 grains of oil of lavender, by the as sistance of gentle heat, and then mixed with 2i grains of lamp-black, and half a grain of indigo : or 120 grains of oil of lavender, 17 grains of copal, and 60 grains of vermilion. A little oil of lavender, or of turpentine, may he added, if the ink be found. two thick. Mr Sheldrake sug gests, that a mixture of genuine asphal tum, dissolved in oil of turpentine, am ber varnish, and lamp-black, would be still superior.
When writing with common ink has been effaced by means of oxygenated muriatic acid, the vapour of sulphuret of ammonia, or immersion in water impreg nated with this sulphuret, will render it again legible. Or if the paper that con tained the writing be put into a weak so lution of prussiate of potash, and when it is thoroughly wet, a sulphuric acid be added to the liquor, so as to render it slightly acidulous, the same purpose will be answered.
Mr. Haussman has given some compo sitions for marking pieces of cotton or li nen, previous to their being bleached, which are capable of resisting every ope ration in the processes both of bleaching and dyeing, and consequently might be employed in marking linen for domestic purposes. One of these consists of asphal tum dissolved in about four parts of oil of turpentine, and with this is to be mixed lamp-black, or black lead in fine powder, so as to make an ink of a proper consist ence for printing with types. Another, the blackish sulphate left after expelling oxygen gas from oxide of manganese with a moderate heat, being dissolved and filtered, the dark grey pasty oxide left on the filter is to be mixed with a very little solution of gum tragacanth, and the cloth marked with this is to be dipped in a so lution of potash or soda, mild or caustic, in about ten parts of water.
Among the amusing experiments of the art of chemistry, the exhibition of sym pathetic inks holds a distinguished place.
With these the writing is invisible, until some reagent gives it opacity. We shall here mention a few out of the great num ber, that a slight acquaintance with che mistry may suggest to the student. 1. If a weak infusion of galls be used, the writ ing will be invisible till the paper be moistened with a weak solution of sul phate of iron. It then becomes black, because these ingredients form ink. 2. If paper be soaked in a weak infusion of galls, and dried, a pen dipped in the so lution of sulphate of iron will write black on that paper, but colourless on any other paper. 3. The • luted solutions of gold, silver, or mercury, remain colourless up on. the paper, till exposed to the sun's light, which gives a dark colour to the oxides, and renders them visible. 4.
Most of the acids or saline solutions, be ing diluted, and used to write with, be come visible by heating betbre the fire, which concentrates them, and assists their action on the paper. 5. Diluted prussiate of potash affords blue letters, when wet ted with the solution of sulphate of iron. 6. The solution of cobalt in aqua-regia, when diluted, affords an ink which be comes green when held to the fire, but disappears again when suffered to cool. This has been used in fanciful drawings of trees, the green leaves of which ap pear when warm, and vanish again by cold. This effect has not been explain ed. If the heat be continued too long af ter the letters appear, it renders them permanent. 7. If oxide of cobalt be dis solved in acetous acid, and a little nitre added, the solution will exhibit a pale rose colour when heated, which disap pears on cooling. 8. A solution of equal parts of sulphate of copper and muriate of ammonia gives a yellow colour when heated, that disappears when cold.
Sympathetic inks have been proposed as the instruments of secret correspond ence. But they are of little use in this respect, because the properties change by a few days remaining on the paper ; most of them have more or less of a tinge when thoroughly dry ; and none of them resist the test of heating the paper till it begins to be scorched.