1. The paper is at first warmed: if the ink used is of the first class, the characters will now become legible; otherwise the examination is continued as below.
2. The paper is exposed to the action of steam, in order to moisten the ink present (care being taken to avoid dissolving the characters), and a current of sulphuretted hydrogen allowed to act upon it. If the ink used consists of a lead, bismuth, or gold salt, a black coloration will ensue; if salts of cadmium or arsenic were employed, the characters will acquire a yellow color; if, finally, a salt of antimony was used, a red coloration will be produced.
3. If no coloration was caused by the action of sulphuretted hydrogen, it is probably that either a solution of ferrocyanide of potassium or a persalt of iron has been resorted to. Each of these solutions is separately applied on a small portion of paper by means of a brush, and notice taken if the characters become visible. The solution that produced the change is then applied over the entire sheet.
4. In case only negative results were obtained in the preceding operations, it must not yet be concluded that a sympathetic ink has not been used, although we are left without further recourse to chemical tests. Numerous organic compounds may have been resorted to, the detection of which is almost impossible; moreover, if a mistake was made in regard to the preparation supposed to have been used, the reagents employed for its detection may render the discovery of another ink absolutely impossible. It is therefore often necessary to apply mechanical tests. For this purpose, the paper is spread upon a glass plate, uniformly moistened with water, and a second plate placed over it: if the characters were written with a pulverulent substance suspended in water or mucilage, they may often be observed upon examining the transparency of the paper. In case the substance used is both colorless and soluble, the detection of the written characters will be more difficult; still, indelible traces may possibly have been left by the pen. If, however, the ink employed is a colorless and transparent organic compound of rare occurrence, and was applied with a fine pencil-brush which failed to affect the paper, it must be acknowledged that little or nothing can be definitely determined as to its presence or absence.
FALSIFICATION OF COINS AND ALLOYS.
In all civilized countries a fixed standard for coins and precious alloys is established by law, in order to prevent the perpetration of frauds which would be of serious injury to the public welfare. The substitution of coins consisting of an alloy inferior in value to the standard fixed by law, is too advantageous a fraud not to be often attempted.
Coins are most frequently altered by clipping; by stuffing, that is, by boring the coin and inserting an alloy of small value; by doubling, which operation consists in covering its face with two thin laminæ taken from a genuine coin; and by applying a coating of gold or silver by means of electro-plating.
In order to ascertain if a coin has been counterfeited, its weight should at first be determined. If it has been clipped, or consists of an alloy possessing a density less than that of silver or gold, the fact is immediately demonstrated by its decreased gravity.
The coin is further tested by throwing it down upon a hard substance: gold and silver give a ringing sound, whereas the majority of other metals produce a dull sound.
The result obtained by this latter test often fails to be reliable. A skilful counterfeiter may prepare an alloy equally sonorous and heavy as silver or gold; in fact, M. Duloz exhibited to the author an alloy, prepared by him, possessing the density, sonorousness and lustre of silver; the composition of which, for obvious reasons, has not been published.