He sends the rain-water from the floor of the vault of heaven and there it is conveyed along channels of various capacities in due measure. Where the torrent flows, it drives before it unworthy matter and impurities floating on the surface gathering into a heap of froth resembling aptly the foamy admixture and impurities mounting to the surface of the heated ores for the separation of precious metals used for ornaments and other uses. Thus does Allah give examples with allegorical representations of truth and falsehood. As to the froth and the scum, they are rejected and disappear, whereas what is profitable to the people remains behind on earth. Thus does Allah describe a subject under the guise of some other subject of aptly suggestive resemblance.