Earth and alkalies do not act on gold in the dry way. Sulphur, which com bines with most metals, has no effect on this. A process, called dry-parting, is grounded on this property ; and is more especially used in separating silver from gold, when the quantity of the latter metal is too small to answer the expense of dissolving the larger mass of silver in nitric acid. For this purpose the mixed metal is fused, and flowers of sulphur thrown on its surface. These combine with the silver in the form of a black sco ria, while the gold remains at the bottom in its metallic state. The operation of dry-parting does not leave the gold in a state of purity ; because the last portions of silver are defended from the action of the sulphur. But when the quantity of silver is thus diminished, the operation of parting with aqua-fortis, or nitric acid, may be advantageously used.
Sulphuret of potash dissolves gold in the dry way. Equal parts of sulphur and potash are hastily fused with one-fourth of a part of gold leaf. This combination is soluble in water, with which it forms a yellowish green solution. By the addi tion of an acid, the gold is thrown down in combination with the sulphur, of which it may be deprived by heat.
Most metals unite with gold by fusion. With silver it forms a compound, which is paler, in proportion to the quantity of silver added. It is remarkable that a cer tain proportion, for example, a fifth part, renders it greenish. From this circum stance, as well as from that of a consider able proportion of these metals separating from each other by fusion, in consequence of their different specific gravities, when their proportions do not greatly differ, it should seem that their union is little more than a mere mixture without combina tion ; for, as gold-leaf transmits the green rays of light, it will easily follow that par ticks of silver, enveloped in particles of gold, will reflect a green instead of a white light.
A strong heat is necessary to combine platina with gold : it greatly alters the colour of the gold if its weight exceed the forty-seventh part of the mass. Mr. Francillon, however, informs us, that six parts of gold and one of malleable platina produce a metal of a beautiful colour, great malleability, susceptible of a fine polish, and more unalterable than gold itself. It does not much affect the duc tility. The Spanish ministry has prohibit ed the exportation of platina from Ame rica, lest it should be used in adulterating gold ; but this does not appear to be a danger which need be feared, as chemis try has long been in possession of several simple and expeditious methods of detect ing this fraud, which besides is evident to the sight when the quantity of de basement is considerable. It may be
questioned, likewise, whether the value of platina would not soon equal that of gold, if its properties and uses were bet ter known in society. Gold made stand ard by platina, and hammered, is tolera bly elastic.
Mercury is strongly disposed to unite with gold, in all proportions with which it forms an amalgam : this, like other amalgams, is softer, the larger the pro portion of mercury. It softens and liquifies by heat, and crystallizes by cooling.
Lead unites with gold, and considers. my impairs its ductility, one-fourth of a grain to an ounce rendering it complete ly brittle. Copper renders gold less duc tile, harder, more fusible, and of a deep er colour. This is the usual addition in coin, and other articles used in society.— Tin renders it brittle in proportion to its quantity ; but it is a common error of chemical writers, to say that the slightest addition is sufficient for this purpose.— 'W hen alloyed with tin, however, it will not bear a red heat. With iron it forms a grey mixture, which obeys the magnet. This metal is very hard, and is said to be mucWsuperior to steel for the fabrication of cutting instruments. Bismuth renders 0-old white and brittle ; as do likewise nickel, manganese, arsenic, and antimo ny. Zinc produces the same effect ; and, when equal in weight to the gold, a me tal of a fine grain is produced, which is said to be well adapted to form the mir rors of reflecting telescopes, on account of the fine polish it is susceptible of, and its not being subject to tarnish. The al loys of gold with molybdena are not . known. It could not be mixed with tung sten on account of the infusibility of this last substance. Mr. Hatchett gives the following order of different metals, ar ranged as they diminish the ductility of gold : bismuth, lead, antimony, arsenic, zinc, cobalt, manganese, nickel, tin, iron platina, copper, silver. The first three were nearly equal in effect ; and the pia tina was not quite pure.
For the purpose of coin, Mr. Hat chett considers an alloy of equal parts of silver and copper as to be preferred, and copper alone is preferable to silver alone.