Secret or sympathetic inks are invisible until the writing is subjected to a subsequent operation, such as warming or exposing to sunlight. To further aid the object in view, the paper may be first steeped in a liquid and the writing only made visible by using another liquid which has some chemical affinity with the previous one. The number of this kind were but few but have multiplied as chemistry progressed. The ancients were acquainted with several modes. Ovid indiscreetly advises the Roman wives and maidens if they intend to make their correspondence unreadable to the wrong persons to write with new milk, which when dried may be rendered visible by rubbing ashes upon it or a hot iron. Pliny suggests milky juices of certain plants of which there are a considerable variety. Indelible ink is not used for writing purposes on paper, but is found best adapted for marking linen and cancellation or endorsing purposes. It is chiefly composed of nitrate of silver preparations, to which heat must be applied after it has been dried; or a pigment is commingled with the same vehicles used in making common printing ink and in its use treated as such. Diamonds, gold, silver, platinum and a host of other materials are manufactured into ink and are to be placed under the head of miscellaneous inks. They are in great number and of no interest in respect to ink writing except for engrossing or illuminating. Still another ink once held in much esteem and now almost obsolete is the so-called "safety" ink. Manufacturers, chemists and laymen in great number for many years wasted money, time and energy in diligent worship at a secret shrine which could not give the information they sought. A summary of the meager and barren results they secured is of little value and unimportant. Hence, there is no REAL "safety" ink. It is true that lampblack (carbon) as made into ink, resists any chemical or chemicals, but simple water applied on a soft sponge will soon remove such ink marks. The reason for this is obvious, the ink does not penetrate the paper. "Safety" ink which will not respond to acids may be affected by alkalis, or if resisting them separately, will yield to them in combination. CHAPTER XV. OFFICIAL AND LEGAL INK. FIRST COMPLETE OFFICIAL INVESTIGATION OF INK IN THIS COUNTRY--THE HONOR DUE TO ROBERT T. SWAN OF BOSTON--RESUME OF HIS REPORTS TO THE LEGISLATURE OF THE STATE OF MASSACHUSETTS--THE SWAN LAW ADOPTED IN 1894 BY THE STATE OF MASSACHUSETTS--UNITED STATES TREASURY DEPARTMENT ADOPTS AN OFFICIAL INK IN 1901--UNSUCCESSFUL ATTEMPT TO SECURE INK LEGISLATION IN THE STATE OF NEW YORK--COMMENTS OF THE PUBLIC PRESS OF THAT PERIOD--DIFFERENT WORKS WHICH MORE OR LESS DWELL ON THE SUBJECT OF INK FROM 1890 TO 1900--CITATIONS FROM ALLEN'S COMMERCIAL ORGANIC ANALYSIS--REFERENCE TO PAPER ABOUT INK READ BEFORE THE NEW YORK STATE BAR ASSOCIATION.