vert into chloride of silver just 184.25 parts of pure silver; consequently the quantity of salt necessary to convert into chloride 1000 parts of silver is 542.74, as found by the proportion—

184.25 100 :: 1000: 542.74.

A standard solution of salt is accordingly so prepared as that a given measure (the French decilitre) shall contain 542.74 thousandths of a gram of salt. The normal weight employed for silver assays is the gram, (equal to about 15.4 troy grains,) which is marked 1000, and has its subdivisions, in practical weighings, to the half or quarter thousandth.

Besides this standard solution, which effects the main precipitation of chloride of silver, there is a decimal solution, of one-tenth the proportion of salt, which it is expedient to use for the lesser and final precipitations.

In the mode of assay under consideration, it is necessary that the portion of alloyed silver used shall contain as nearly as may be 1000 parts of pure silver. The rigid standard requires that of 1000 parts by weight 900 shall be of pure silver; but the law allows a variation from this ratio, provided that it do not exceed three thousandths. The fineness may, therefore, be as low as 897, and as high as 903. In the practice of the assay, it is found most convenient to assume the lower extreme. Now, the weight of metal of the fineness 897, which would contain 1000 parts of silver, is 1114.83, as found by the proportion

897: 1000 :: 1000: 1114.83.

The nearest integer to this number is employed, and the weight of metal taken for the assay is 1115.

PROCESS OF ASSAY.

The reserved silver coins are melted together in a black-lead crucible, with the addition of fine charcoal within the pot, to prevent oxidation, and to allow of dipping out. After stirring, a small portion of the fluid metal is poured quickly into water, producing a granulation, from which the portion for assay is taken. As this differs from the mode pursued with gold, it must be specially noted that in the case of silver alloyed with copper there is a separation, to a greater or less degree, between the two metals in the act of gradual solidification. Thus an ingot cooled in a mould, or any single coin cut out of such ingot, though really 900 thousandths fine on the average, will show such variations, according to the place of cutting, as might even exceed the limits allowed by law. This fact has been established by many experiments, both in this mint and the mint of Paris, since the enactment of our mint law; and it possesses the stubbornness of a law of chemistry. But the sudden chill produced by throwing the liquid metal into water yields a granulation of entirely homogeneous mixture, and it can be proved that the same fineness results, whether by assaying a single granule, or part of one, or a number together.

From this sample the weight of 1115 thousandths is taken, which is dissolved in a glass bottle with nitric acid.

Into this solution the large pipette-full of standard solution of salt is introduced, and it produces immediately a white precipitate, which is chloride of silver, and which contains, of the metallic silver, 1000 parts.

To make this chloride subside to the bottom of the vessel and leave the liquid clear, it is necessary that it be violently shaken in the bottle; and this is accordingly done by a mechanical arrangement, for the necessary time.

Unless the coins have chanced to be below the allowable limit of standard, the liquid will still contain silver in solution, and accordingly a portion of the decimal solution is introduced from the small pipette, capable of precipitating a

thousandth of silver, and a white cloud of chloride will show itself. More doses are added if the indications require it.

The liquid is again shaken and cleared; and the process is thus repeated until the addition of the salt water shows only a faint trace of chloride below the upper surface of the liquid.

Let us suppose, for the sake of an example, that three measures of the decimal solution have been used with effect. This will show that the 1115 parts of the coin contained 1003 of pure silver; and thus the proportion of pure silver in the whole alloyed metal is ascertained.

TEST ASSAY.

For the foregoing process to be exact, it is necessary that the saline solution be of the true standard strength, or be such that the quantity of it, measured in the large pipette, shall be just sufficient to precipitate 1000 parts of silver. This cannot be assumed without proof, and a test assay is accordingly made as follows:

A roll of silver, known to be of absolute purity, is kept from year to year, in an envelope, under the seal of the chairman of the assay commissioners. This being opened in their presence, a portion of the silver is taken, and 1004 parts carefully weighed off and submitted to the process of assay described above. If the salt water used be of the exact standard, it is evident that as the solution in the larger pipette will precipitate 1000 parts of silver, four measures of the decimal solution will be required to precipitate the remaining four parts.

But as the normal or standard solution is affected, from day to day, by changes of temperature or other influences, the finishing decimal doses may be more or fewer; and the other assays are to be corrected by the proof-piece accordingly.

CALCULATION OF FINENESS.

By the assay thus corrected, the number of parts of silver contained in 1115 of the metal under trial is ascertained; and the fineness, in thousandths, is then found by the proportion: As 1115 is to the number of parts of fine silver, so is 1000 to the fineness of the alloyed silver, in thousandths.

Thus, if the assay show the presence 1005 parts of fine silver, the fineness of the alloyed silver will be 901.8 thousandths, as found by the proportion

1115 1005.5 :: 1000 : 901.79.

It is on this principle that the following table is constructed. The numbers at the top and the fractions at the side correspond to the measures of the decimal solution used, corrected by the test assay. The numbers in the body of the table show the corresponding fineness of the assay-piece, of which the weight was 1115 parts:

In the testing of single pieces it is to be expected that any gold coin, or a cut from any part thereof, will conform faithfully to the bounds prescribed by law. But the silver coins, in addition to the source of error already pointed out, (the manner of taking assay samples,) are somewhat liable to show too high a result, from several causes. At certain grades of alloy, and especially the standard of 900, the gradual cooling of ingots will draw the better metal to the interior, and the worst towards the exterior and the edges. Hence the fineness of pieces cut off the central part of the ingot is higher than the average fineness of the ingot. Again, in casting ingots from a melting-pot the exposure of the metal to the air during all the time of dipping out, and at the same time the increase of heat towards the bottom of the pot, unavoidably produces a progressive refining, so that the lower ingot is of a higher quality than the average of the whole melt; and, of course, a coin cut from it will be higher still. Yet with the precautions observed, our silver coins should very rarely exceed the superior limit assigned by law; and there is no good reason why they should fall below the legal limit, unless it be the taking of an unfair sample for assay.

A STATEMENT OF FOREIGN GOLD AND SILVER COINS, PREPARED BY THE DIRECTOR OF THE MINT, TO ACCOMPANY HIS ANNUAL REPORT, IN PURSUANCE OF THE ACT OF FEBRUARY 21, 1857.

EXPLANATORY REMARKS.

The first column embraces the names of the countries where the coins are issued; the second contains the names of the coin, only the principal denominations being given. The other sizes are proportional; and when this is not the case, the deviation is stated.

The third column expresses the weight of a single piece in fractions of the troy ounce carried to the thousandth, and in a few cases to the ten thousandth, of an ounce. The method is preferable to expressing the weight in grains for commercial purposes, and corresponds better with the terms of the mint. It may be readily transferred to weight in grains by the following rules: Remove the decimal point; from one-half deduct four per cent. of that half, and the remainder will be grains.

The fourth column expresses the fineness in thousandths, i. e. the number of parts of pure gold or silver in 1000 parts of the coin.

The fifth and sixth columns of the first table express the valuation of gold. In the fifth is shown the value as compared with the legal contents, or amount of fine gold in our coin. In the sixth is shown the value as paid in the mint, after the uniform deduction of one-half of one per cent. The former is the value for any other purposes than re-coinage, and especially for the purpose of comparison; the latter is the value in exchange for our coins at the mint.

For the silver. there is no fixed legal valuation, the law providing for shifting the price according to the condition of demand and supply. The present price of standard silver is 122 cents per ounce, at which rate the values in the fifth column of the second table are calculated. In a few cases, where the coins could not be procured, the data are assumed from the legal rates, and so stated.