A good illustration of the dangers of the argumentum ex silentio is furnished by the fact that salt, the most necessary of minerals, is never once mentioned in the Rigveda. And yet the Northern Panjab is the very part of India where it most abounds. It occurs in the salt range between the Indus and the Jhelum in such quantities that the Greek companions of Alexander, according to Strabo, asserted the supply to be sufficient for the wants of the whole of India. Among the metals, gold is the one most frequently mentioned in the Rigveda. It was probably for the most part obtained from the rivers of the north-west, which even at the present day are said to yield considerable quantities of the precious metal. Thus the Indus is spoken of by the poets as "golden" or "having a golden bed." There are indications that kings possessed gold in abundance. Thus one poet praises his royal benefactor for bestowing ten nuggets of gold upon him besides other bountiful gifts. Gold ornaments of various kinds, such as ear-rings and armlets, are often mentioned. The metal which is most often referred to in the Rigveda next to gold is called ayas (Latin, aes). It is a matter of no slight historical interest to decide whether this signifies "iron" or not. In most passages where it occurs the word appears to mean simply "metal." In the few cases where it designates a particular metal, the evidence is not very conclusive; but the inference which may be drawn as to its colour is decidedly in favour of its having been reddish, which points to bronze and not iron. The fact that the Atharva-veda distinguishes between "dark" ayas and "red," seems to indicate that the distinction between iron and copper or bronze had only recently been drawn. It is, moreover, well known that in the progress of civilisation the use of bronze always precedes that of iron. Yet it would be rash to assert that iron was altogether unknown even to the earlier Vedic age. It seems quite likely that the Aryans of that period were unacquainted with silver, for its name is not mentioned in the Rigveda, and the knowledge of silver goes hand in hand with that of iron, owing to the manner in which these metals are intermingled in the ore which produces them. These two metals, moreover, are not found in any quantity in the north-west of India.