Sixth Operation.

FOR every ten pounds of yarn make a ley from half a pound of pot or pearl ashes; pour the clear ley into the boiling-pan; add a sufficient quantity of water thereto that will cover the yarn about four inches; light the fire, and enter the yarn, when the liquor is a little warm; observe to keep it constantly under the liquor for two hours; increase the heat regularly till it come to a scald; then take the yarn out, wash it, and hang it to dry as in former operations.

Seventh Operation.

MAKE a sour liquor of oil of vitriol and water; the degree of acidity may be a little less than the juice of lemons; lay the yarn in it for about an hour, then take it out, wash it very well and wring it; give it a second washing and wringing, and lay it on a board.

N. B. This operation is to dissolve the metallic particles, and remove the ferruginous matter that remains on the surface of the thread after the fifth operation.

Eighth Operation.

FOR every ten pounds of yarn dissolve one pound of best white soap in clear water, and add as much water to this liquor in your boiling-pan as will be sufficient to boil the yarn for two hours. When these liquors are well mixed light the fire, enter the yarn, and bring the liquor to boil in about an hour. Continue it boiling slowly an hour; take it out, wash it in clear water very well, and hang it to dry as in former operations: when dry it is ready for the weaver.

N. B. It appears to me, from experiments that I have made, that less than four operations in the preparation of the yarn will not be sufficient to cleanse the pores

of the fibres of the cotton, and render the colour permanent.

No. 85.

Observations on the Employment of Platina in Porcelain Painting. By Professor KLAPROTH.*

IN the course of half a century since platina was introduced and known in Europe, the experiments made with it by various eminent chemists seem to have exhausted every thing that relates to the physical and chemical properties of this remarkable metal. The imperfect information, however, which relates to its mineralogical and natural history seems to require further investigation, though it must at the same time be acknowledged that our information in this respect appears to be worthy of confidence, as the Spanish government has entrusted the inspection and management of its mines in South America to men who to a knowledge of mineralogy and mining unite great zeal for the improvement of these sciences.

The real origin of platina is in all probability to be ascribed to revolutions which have taken place in the Cordilleras by volcanoes, earthquakes, and inundations; and it is not improbable that these mountains still contain in their interior parts entire veins of platina, the discovery of which is perhaps reserved for future times.

At present, Peru is the only known country where platina is found, and particularly the district of Choco, where it is collected in the valleys between the moun

Tilloch, vol. 17, p. 135. From Scherer's Allgemeines Journal der Chemie, no. 52, 1802.

tains and rivers along with the gold in small laminæ, or it is obtained by washing the earth.* When the largest grains of gold have been picked from the mixed mass of gold and platina, the remaining gold is extracted by amalgamation; by means of which operation the platina is left behind in the form of flat plates or scales.

The deceptions formerly practised by mixing gold with platina have induced the Spanish government to prevent the exportation of it, and to give orders to all their servants in that country to keep the platina by them, and to wash it in water from time to time. But as means have been found to detect easily and in a certain manner the adulteration of gold with platina, and also to employ it for valuable purposes, it is to be hoped that the Spanish government will not persist in causing a prohibition so injurious to the arts and to its own finances to be executed with the former se verity.

My object at present is not to enlarge on the chemical and physical properties of platina, but only to of fer a few observations on the uses in the arts to which it has hitherto been applied; and then to give an account of the result of an experiment which I made in regard to a new application of this metal to objects of manufacture.

The apparent infusibility of platina by itself, formerly considered as an insuperable obstacle, was sufficient to prevent the employment of it except in combination with other metals, as experience showed that it was capable of uniting with the greater part of them by fusion. Of such mixtures, that arising from a combination of brass and platina was found to be exceedingly proper for the specula of reflecting telescopes, as this alloy was

* Platina is found in Spain, and also in St. Domingo.

susceptible of a beautiful polish not subject to be injured by the prejudicial influence of the atmosphere and of moisture. At first, however, the employment of platina was not extended further until the experiments made known by Morveau, Sage, and other chemists, and afterwards prosecuted on a larger scale by count Von Sickingen, formed as it were an epoch in the history of this metal, and showed in what manner platina might be freed from its foreign particles, be welded, hammered, and drawn out into wire, so as to be applicable to a variety of purposes.

It was, however, not yet possible to employ it in cases which required an actual fusion for the purpose of casting it; because this metal, in its purified state, was always by itself infusible in a common furnace. It was therefore a discovery of great importance to find that platina may be rendered fusible by arsenic; and that when mixed with this substance it may be cast in moulds, while the volatile metal employed as a flux may be again driven off by heat, so that the cast platina may then be hammered like any other metal. By employing this method, first made known by my worthy colleague M. Achard, vessels and articles of various kinds are made of platina, and particularly at Paris.

Bergman, however, had shown that platina which could be reduced to a state of fusion only by employing a large burning mirror, might be fused also by means of oxygen gas. In this manner M. Pelletier, by means of phosphoric glass, made from bones, combined with charcoal powder, brought large masses of platina to a state of complete fusion.

How far platina might be employed in porcelain painting has never yet, as far as I know, been examined: I therefore thought it of considerable importance to make some experiments on this subject, which did not deceive

my expectation; but, on the contrary, convinced me that this object, in the hands of an ingenious artist, may be brought to perfection.

Gold and silver have hitherto been the only metals susceptible of being employed in their metallic form in painting and ornamenting porcelain, glass, and enamel. Gold answers this purpose so completely, that nothing further can be wished for on this head; whereas silver does not answer so well. As it possesses less density and is more porous than gold, it does not cover the ground so completely when applied to porcelain in thin leaves. The second cause of the inferiority of silver when employed in painting on porcelain arises from its nature, in consequence of which, when exposed to sulphureous and other phlogistic vapours, it becomes tarnished, loses its metallic splendour, and at length grows black. This inconvenience renders silver unfit for being employed in fine porcelain painting, and confines the application of metallic substances in this manner to gold alone.

Platina, in this respect, may be classed next to gold; and by its white colour may supply the place of silver without possessing any of its faults. It is not only capable, on account of its density and weight, in which it exceeds gold, of covering the ground completely, without leaving any perceptible interstices, as silver does; but it withstands like gold all the variations of the atmosphere, as well as sulphureous and other vapours.

The process which I employ in the application of platina to painting on porcelain is simple and easy: it is as follows:-I dissolve crude platina in aqua regia, and precipitate it by a saturated solution of sal ammoniae in water. The red crystalline precipitate thence produced is dried, and being reduced to a very fine powder