Although borax has long been known as an article of traffic, there is scarcely any production with the origin of which we have been, till lately, less acquainted. It is found in a native, though impure state, in a mountain lake, situated about fifteen days’ journey from the capital of Thibet in the East Indies. This lake is so encompassed with hills as to have no stream either falling into it or flowing from it. The water is salt to the taste, and contains both borax and common salt; and the edges and shallow parts are covered with a stratum of this substance, which is dug up in considerable masses for exportation. It has here the name of _tinkal_, and is usually brought into Europe enveloped in a kind of fatty substance. The mode of refining it was for a long time kept, by the Dutch and Venetians, amongst those secrets which a want of sufficient research alone prevented from being generally known. When refined, it is called _borax_. The uses of borax are numerous. It is employed as a flux for metals, being found to produce a more perfectly limpid fusion than any other substance. For the same reason it is made an ingredient in the finest kinds of glass, and particularly in some of the coloured glass pastes which are manufactured in imitation of gems. But its chief use is to jewellers and goldsmiths, to facilitate the soldering of gold and silver. Borax is also used in medicine. 205. POTASH FAMILY. Potash is an alkaline substance (42), of white colour, and of smell somewhat resembling that which is perceived during the slaking of quick─lime (137). It is extremely corrosive, and remarkably acrid to the taste. In a mineral state it is found only in combination with nitric acid (30). Potash principally exists under the form of a salt, in vegetable substances; and is obtained by burning them, afterwards repeatedly washing the ashes with water, and then filtering and evaporating these to dryness. The appellation of potash was given to this salt from its having formerly been prepared in large iron pots. The uses to which it is applied are numerous. In chemistry it is employed for a variety of purposes; and also in many arts and manufactures, in scouring, washing, bleaching, dyeing, glass─making, and several others. Its corrosive property is such that it is often used by surgeons under the name of _potential cautery_, to open abscesses, and to destroy useless or hurtful excrescences. =Potash=, after it has been made red hot, is rendered whiter and more pure. In this state it has the name of pearl ash. 206. _NITRE, or SALTPETRE, is a salt which consists of potash in combination with nitric acid (30)._