per, and subsequently with alcohol, until no trace of free sulphuric acid remained. After drying, the precipitate of sulphate of lead and gold, with the filter cut into small pieces, was transferred to a beaker glass, and the whole was then digested with pure bicarbonate of soda * (prepared from the oxalate), during at least forty-eight hours. The sulphate of soda formed by this operation was then filtered off from the carbonate of lead and metallic gold, and the amount of sulphuric acid which it contained determined by precipitation with chloride of barium in the usual way. From the amounts of sulphate of baryta thus obtained, recorded in column (3) of the table, the amounts of lead (col. 4) in the mixed precipitates of sulphate of lead and gold were

calculated.

The carbonate of lead, mixed with bits of paper and gold, above mentioned, was now dissolved in dilute nitric acid, the solution evaporated with sulphuric acid until all the nitric acid had been expelled, subsequently treated with water acidified with sulphuric acid, and, after standing for forty-eight hours, collected upon a tared filter, washed with dilute sulphuric acid and alcohol, dried at 100°, and weighed. The results may be found in column (a) of the table.

The re-agents used in the foregoing operations were all chemically pure. A special experiment was moreover made, in which portions .of the nitric acid, ammonia, chloride of ammonium, and sulphuric acid used, twice as large as any which were actually employed in the analysis of either of our samples of coin, were evaporated together, and subjected to a course of treatment identical with that which the solutions of coin were forced to undergo; but not a trace of sulphate of lead, or of any precipitate, other than a few light and utterly insignificant floating flocks, probably alumina, could be detected as the result of this trial.

That our process is sufficiently accurate to exhibit all the lead which the samples of coin examined really contained, we do not think probable. All of the sources of error to which the process, when properly conducted, is subject, tend to cause a slight loss of lead. Thus, in the first place, it is scarcely possible that the last traces of chloride of lead can be removed by washing from the caseous precipitate of chloride

* See Fresenius, Anleitung zur Quantitativen Analyse, (Braunschweig, 4te Aufl. 1858,) p. 286, § 132, II. b. ß.

of silver,* which precipitate is in enormous mass as compared with the amount of chloride of lead; and again, the first precipitate of sulphate of lead, mixed with gold, which is obtained after evaporation with sulphuric acid, occurs in so finely divided a state, that it is exceedingly difficult to prevent loss by the passing of a portion of it through the pores of the filter upon which the precipitate is collected. To one unused to the process, it might seem as if the last traces of sulphuric acid could hardly be removed from this precipitate by washing with alcohol. We have satisfied ourselves, however, that this is easily effected, and that this seeming source of error is, in reality, entirely insignificant; at any rate, the results given in columns (a), (b), and (c) are in no way influenced by this imaginary error. It may not be amiss to observe, that we have been particularly scrupulous with regard to the treatment of the above-mentioned precipitate, having in every instance continued to wash it with alcohol long after any reaction of sulphuric acid could be detected. It should also here be stated, that the results in which we ourselves place the most confidence are those obtained by determining the amount of sulphuric acid in the mixed precipitate of sulphate of lead and gold (see cols. (3), (4), and (5) of the table), for columns (a), (b), and (c) contain the results of a secondary process liable to all the accumulated errors of a long series of operations; moreover, the final precipitate of sulphate of lead is very finely. divided, and, like the first precipitate, liable to pass through the pores of the filter, as has been previously explained. This second series of experiments has been carried out, not from any expectation that the results would closely agree with those of the first series, but merely to control the latter to a certain extent, and to remove any objection which might be made to the indirect method by which they were obtained. Since the possible inaccuracies of the process tend to a loss of lead, we are confident that the amounts of lead we have obtained are in no instance greater than those really existing in the coin.

The precipitates of pure sulphate of lead (column (a) of the table) were, in every instance, reduced on charcoal with carbonate of soda, and the button of lead obtained carefully tested for silver, of which none could be detected in any case; nor did the metal exhibit any reactions, other than those of pure lead.

The residue of gold and filter paper left after dissolving the car

* Compare Mulder, op. cit., p. 217.

bonate of lead in nitric acid for the final lead determination, was in each case incinerated, and then fused with carbonate of soda before the blowpipe. A distinct button of metallic gold was obtained in every instance. The amount of gold obtained varied, however, to a very considerable extent: it was abundant in the American and Spanish coin, less was observed in the English and Mexican, and but little in the French coin or the American fine silver.*

The silver of the French coin was undoubtedly prepared by the sulphuric-acid process,† in which the granulated alloy of silver, gold,

* Besides the quantitative analyses recorded above, which have been made by this method, it should be mentioned, that in several qualitative experiments upon American coins, by the method as described or slightly modified, indications of lead were obtained in every instance. The same remark is true of two qualitative analyses, one of a dime and another of a quarter-dollar, made as follows: the coin having been dissolved in nitric acid, and the insoluble black residue of sulphide of silver and a little gold separated by filtering, the solution was mixed with pure acetate of soda (see Fresenius, op. cit., p. 428, § 163, 1. y) to prevent precipitation of chloride of lead in subsequent operations, and the whole heated over the water-bath; the silver was then precipitated with dilute chlorhydric acid, and the chloride of silver collected upon a filter and washed. Through the filtrate sulphuretted hydrogen was passed, the mixed sulphide of copper, lead, and gold collected upon a filter, treated with nitric acid, the acid solution evaporated to dryness, the residue taken up with water, and the solution filtered; in the filtrate lead was detected, while the residue contained gold.

With regard to this residue of gold, which has also been alluded to in the text, it should be remarked that it has been entirely neglected by many analysts who have published determinations of the amounts of gold in various silver coins. Indeed, it would appear as if the fact were not generally known to chemists, that nitric acid can dissolve a little gold (as it can platinum), when an alloy containing a small quantity of gold is treated with this acid (compare Mulder, op. cit., p. 159). Another common error of analysts has been, to class as "gold" the dark-colored residue which is usually left when silver coin is dissolved in dilute nitric acid. This precipitate does, in fact, generally contain a little gold, though in our own experiments by far the larger portion of this metal passed into solution in the nitric acid, and was separated at a subsequent stage of the analysis, as has been described; Gay Lussac (Ann. Ch. et Phys., 1836, [2.] LXIII. 334) long ago showed that it is principally composed of sulphide of silver; to which fact attention has also been called by Mulder, in his excellent little treatise, just cited, p. 178.

† See D'Arcet in Dingler's Polyt. Journ., 1828, XXVIII. 3; from Annales Mensuelles, Mai, 1827, p. 131. Also, Ure's Dictionary of Arts, &c., 4th edit., (Boston, 1853,) II. 543. Dumas's Traité de Chimie Appliquée aux Arts, (Paris, 1833,) IV. 464.

copper, &c. is dissolved in hot concentrated sulphuric acid, and the silver subsequently precipitated, by means of metallic copper, from a somewhat diluted hot solution of the sulphate of silver thus obtained. Since ordinary commercial sulphuric acid is used in this operation, it is not strange that a portion of the lead with which it is contaminated should be transferred to the silver. The lead salt is perhaps reduced by the metallic copper in the manner so well described by Odling, in his memoir "On the Reciprocal Precipitations of the Metals." It is, moreover, not impossible that some sulphate of lead may fall when the acid liquor is diluted, and become mixed with the precipitated silver. It is not credible, however, that the lead in the coin can have been derived from the copper used to form the standard alloy, for this supposition would imply that copper of very inferior quality had been employed. According to Karsten, copper which contains but one per cent of lead is utterly unfit for manufacturing purposes, since it cannot be worked at any temperature. Now the least amount of lead which we found in fifty cents' worth of American silver is 0.0253 gram. in 10 half-dimes of 1853. These 10 half-dimes contain 1.243 gram. of copper, and if this metal had been the source of the lead, it must have contained more than two per cent of that impurity.

We are ignorant of the process of preparing silver which was in use at the British Mint in 1816. Perhaps the silver in the coin of that date was obtained by cupellation, and it is well known that silver so obtained almost always contains lead. §

* D'Arcet, Journ. für tech. u. œkon. Ch., 1829, IV. 420.

† Quar. Journ. Chem. Soc. of London, 1857, IX. 289.

In his System der Metallurgie, (Berlin, 1832,) V. 245.

§ Authorities differ as to the amount of lead contained in crude cupelled silver ("lightened silver," argent éclairé, Blicksilber). According to Berthier (Essais par la Voie Seche, (Paris, 1848,) II. 724), it contains, on an average, only 1 per cent of lead. Kerl (Handbuch der metallurgischen Hüttenkunde, (Freiberg, 1855,) III. 152), on the other hand, says that it contains from 5 to 10 per cent of impurity; and according to Karsten (System der Metallurgie, V. pp. 200, 201) Blicksilber contains at least 12 per cent of lead, and often more, the proportion of lead to silver being entirely dependent upon the temperature of the cupelling furnace. Since such silver is unfit for use, it is refined by small portions, either by a second cupellation at a high heat, or by melting it in crucibles with saltpetre and borax or some other flux (Kerl, op. cit., III. pp. 181 to 198); but it appears to be impracticable, or at least not advantageous in practice, to remove the last traces of lead from the silver by either of these processes. (See Kerl, op. cit., I. 224; or Plattner, Probirkunst mit dem Löthrohre, (Leipzig, 1853,) p. 403).

The silver of the Spanish and Mexican coins was doubtless prepared by the American system of amalgamation. In this process, speaking in general terms, it would appear that silver is reduced by means of metallic mercury from a solution of chloride of silver in chloride of sodium, the amalgam which is formed being exposed meanwhile to the action of chloride of copper and perchloride of iron.

It is to be inferred from the investigations which we have cited, that under these circumstances the amalgam should be almost entirely free from any contamination with the more strongly electropositive metals.† It would seem, indeed, that the American system of amalgamation furnishes purer silver than is obtained by any of the other processes which are employed upon the large scale.

So far as concerns the occurrence of lead in the silver coin of our own country, it could probably be mainly, if not altogether, avoided, by employing zinc free from lead, such as is manufactured in Pennsylvania, as we have already described in our Memoir.

It is interesting to observe, that the occurrence of lead in some of the silver coins of the ancients ‡ has been thought to indicate “that the process of separating lead and silver was less perfectly executed in the ages of antiquity than is at present the case."§ Yet, in none

* Described by Humboldt in his Essai Politique sur le Royaume de la Nouvelle Espagne, (Paris, 1811,) II. 558. See also Karsten, Abhandlungen der phys. Klasse der Akad. der Wissenschaften zu Berlin, 1828, p. 1; and Karsten u. Dechen's Archiv für Mineralogie, etc., 1829, I. 161; and again, ibid. XXV. 178, and in Dingler's Polyt. Journ., 1852, CXXVI. 357. Compare the subsequent statements of Boussingault, Ann. Ch. et Phys., 1832, [2.] LI. 350.

† In the European system of amalgamation, as practised at Freiberg in Saxony, where the silver is reduced from its chloride by metallic iron instead of quicksilver, a similar degree of purity in the silver is not to be expected. (For analyses of such silver, “ Tellersilber,” see Kerl, op. cit., I. 234.)

‡ Walchner, Schweigger's Journal für Ch. u. Phys., 1827, LI. pp. 204, 205. J. W. Draper, Silliman's Am. J. Sci., 1836, [1.] XXIX. 160. Sarzeau, Journal de Pharmacie, 1839, XXV. 503. Brüel and Hausmann, Karsten u. Dechen's Archiv für Mineralogie, etc, 1844, XVIII. 505; also in J. pr. Chem., XXX. 334. J. W. Mallet, Trans. Royal Irish Acad, 1853, XXII. 319. Brüel and Hausmann refer also to earlier determinations of lead in antique coins by Klaproth (probably in his Beiträge zur chemischen Kenntniss der Mineralkörper, 1795 – 1780, B. VI.), and by Goebel (doubtless in his brochure, Ueber den Einfluss der Chemie auf die Ermittelung der Völker der Vorzeit, etc., Erlangen, 1842) ; but they do not indicate precisely where these analyses are to be found, nor have we any means of ascertaining this point.

§ Brüel and Hausmann, loc. cit., Archiv. p 509; J. pr. Chem., p. 338.