is the ratio of the quartz in the mixture by

weight :

66.65 × 2.60 = 173.29

G. To find the per-centage of gold, add these ratios together, and proceed by the rule of proportion :

633.65 +173.29 806.94

=

806.94 is to 633.65 as 100 is to 78.53

Hence, a mixture of quartz and gold, having the specific gravity of 8.067, contains 78.53 per cent. of gold by weight.

XII.

Assay of Platinum Ores.

THIS metal has as yet occurred in the United States only in gold ores, and even then in the merest traces, and hence, perhaps, it would barely deserve a mention in these pages; but the great interest attached to its occurrence, as so rare a metal all over the world, and its useful application to chemical purposes, has caused me to insert some rules for its assay.

If the ore contain platinum in no larger amount than three or four per cent. of the gold, the former, as already observed in Article V. on quartation, will be entirely dissolved in the nitric acid used on account of the silver. From this solution of the two metals precipitate the silver with common salt, or muriatic acid, as chloride

of silver; filter and wash until the water dropping from the funnel no longer contains any of the platinum solution. This latter evaporate to dryness, after adding sal-ammonia. Wash it with alcohol, (see Gold, art. VIII.) and heat the double chloride of platinum and ammonium to redness, thus producing a spongy mass of pure platinum.

If there be more than three or four per cent. of platinum in the gold, its presence is readily perceived, from various circumstances: thus, in evaporating the lead a higher temperature is necessary than is commonly the case, to make the metal flow and acquire a round form; secondly, the bright light cannot be observed; thirdly, the surface of the button is crystalline or rough, and when large, flat and quite irregular, besides looking dull and having a more or less grayish colour; fourthly, the nitric acid is frequently discoloured; and fifthly, the little roll of gold is not of a

pure gold yellow, but rather inclining towards steel gray.

After having thus recognised a larger quantity of this metal, it becomes necessary, since copper is frequently present, to make a prior test, to ascertain, by cupellation, the amount of the alloy of gold and platinum. After that, two assays should be made; the one, to ascertain the exact conjoint per-centage of the two; the other, inquartation (pure) silver being added, to discover the amount of the gold alone. The difference of the two results gives the amount of platinum. The inquartation silver should not be more than from two and a half to three times the weight of the alloy of gold and platinum; and it is often well to add a certain, accurately weighed quantity of pure gold at the same time, so that the gold may afterward be procured in one connected sheet or piece. This ought particularly to be done, when there is as much as a third the weight of the

gold in platinum, as, for instance, in the platinum grains of the Ural Mountains, which contain about 80 per cent. The button is hammered flat, and proceeded with exactly as gold ores, the platinum. dissolving with the silver in nitric acid. But as it does not do so as easily as the latter, at least when in large quantities, it is necessary to repeat the process from the quartering on once or twice, using silver and lead over again. This should be done until nothing but the silver used is dissolved in the nitric acid, or, in other words, until two assays following one another have produced the same results, a thing that may not occur until the fifth time.

It should be remarked, that it is necessary to add a little more lead for cupellation, than would be done if no platinum were in the ore; and also, that just before the bright flash of light occurs during the operation of quartering, it is well to shake the cupel a little, to make the button