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.
In instances of this nature the fusibility of the coin should be determined, and the result obtained compared with the melting point of the legal alloy, or, this failing, a chemical analysis executed. In order to perform the latter test, the coin under examination is boiled with nitric acid: all metals are dissolved, with exception of gold and platinum, which remain unaltered, and tin and antimony, which are converted respectively into metastannic and antimonic acids. The fluid is filtered, the insoluble residue well washed, and then boiled with hydrochloric acid, which dissolves the metastannic and antimonic acids. The solution is again filtered, and the second residue dissolved in aqua regia. The metals dissolved in the several filtrates are then detected, either by the processes previously given for the detection of metallic poisons, or by the more complete methods contained in works on chemical analysis. This qualitative test is, however, insufficient, in case the falsification consisted in merely diminishing the proportions of the valuable metals contained in the alloy, without changing its qualitative composition: it is then necessary to execute a quantitative estimation of the metals present. As this operation requires considerable practice and the methods employed are to be found in all treatises on quantitative analysis, we will not reproduce them here.