The Muhammadans were also destined to set up a new standard of weight, but before this was accomplished nearly five centuries were to elapse. The period under discussion in this chapter is chiefly interesting for the reappearance of silver in the currency, due to the reopening of commercial relations with Central Asia, and for the successive attempts made by various sovereigns to restore order out of the chaos into which the coinage had fallen during the preceding centuries. The gold and silver currency was rectified by Altamsh and his successors with little difficulty; but the employment of billon for their smaller money was fatal; for the mixture of silver and copper in varying proportions,[46] so liable to abuse, proved in the end unworkable as a circulating medium; and not until Sher Shāh substituted pure copper for billon, and adjusted this to his new standard silver coin, the rupee, was the currency established on a firm basis. [46] The variation is due to the fact that silver and copper only form a homogeneous alloy when mixed in the ratio of 71·89 of the former to 28·11 of the latter. This fact was certainly unknown at this period. Cf. _J.A.S.B._, N.S., XXXV, p. 22, “The Currency of the Pathan Sultans,” by H. R. Nevill. The earliest Muhammadan kingdom in India was set up by ’Imādu-d-dīn ibn Qāsim, in Sind, in A.D. 712, but as it exerted little influence on its neighbours, the insignificant coins issued by its later governors need not detain us. The gates of the North-West were first opened to Muslim invaders by the expeditions of the great Sult̤ān Maḥmūd of G̱ẖ̱aznī between the years A.D. 1001 and 1026. In 1021 the Panjāb was annexed as a province of his dominions, and after 1051 Lāhor became the capital of the later princes of his line, driven out of G̱ẖ̱aznī by the chieftains of G̱ẖ̱or. Here they struck small billon coins with an Arabic legend in the Cufic[47] script on the reverse, retaining the Rājpūt bull on the obverse. Maḥmūd himself struck a remarkable silver _tankah_[48] at Lāhor, called on the coin _Maḥmūdpūr_, with a reverse inscription in Arabic, and his name and a translation of the Kalima in Sanskrit on the obverse. [47] Cufic is the earliest rectilineal form of Arabic script. [48] Tankah is an Indian name applied to coins of various weights and metals at different periods. For example, to the large silver and gold pieces of Nāṣiru-d-dīn Maḥmūd, and later to a special copper issue of the Mug̱ẖal Akbar.