Since the soma sacrifice, beside the cult of fire, forms a main feature in the ritual of the Rigveda, the god Soma is naturally one of its chief deities. The whole of the ninth book, in addition to a few scattered hymns elsewhere, is devoted to his praise. Thus, judged by the standard of frequency of mention, Soma comes third in order of importance among the Vedic gods. The constant presence of the soma plant and its juice before their eyes set limits to the imagination of the poets who describe its personification. Hence little is said of Soma's human form or action. The ninth book mainly consists of incantations sung over the soma while it is pressed by the stones and flows through the woollen strainer into the wooden vats, in which it is finally offered as a beverage to the gods on a litter of grass. The poets are chiefly concerned with these processes, overlaying them with chaotic imagery and mystical fancies of almost infinite variety. When Soma is described as being purified by the ten maidens who are sisters, or by the daughters of Vivasvat (the rising sun), the ten fingers are meant. The stones used in pounding the shoots on a skin "chew him on the hide of a cow." The flowing of the juice into jars or vats after passing through the filter of sheep's wool is described in various ways. The streams of soma rush to the forest of the vats like buffaloes. The god flies like a bird to settle in the vats. The Tawny One settles in the bowls like a bird sitting on a tree. The juice being mixed with water in the vat, Soma is said to rush into the lap of the waters like a roaring bull on the herd. Clothing himself in waters, he rushes around the vat, impelled by the singers. Playing in the wood, he is cleansed by the ten maidens. He is the embryo or child of waters, which are called his mothers. When the priests add milk to soma "they clothe him in cow-garments." The sound made by the soma juice flowing into the vats or bowls is often referred to in hyperbolical language. Thus a poet says that "the sweet drop flows over the filter like the din of combatants." This sound is constantly described as roaring, bellowing, or occasionally even thundering. In such passages Soma is commonly compared with or called a bull, and the waters, with or without milk, are termed cows. Owing to the yellow colour of the juice, the physical quality of Soma mainly dwelt upon by the poets is his brilliance. His rays are often referred to, and he is frequently assimilated to the sun.