Among the Indians the collection of their old songs and forms is known as the Veda, _i.e._ knowledge: it forms the knowledge of the priest. We possess these songs in three groups. The oldest, and no doubt the original group, the Rigveda, _i.e._ the knowledge of thanksgiving, comprises in ten books more than a thousand of the traditional poems and sacrificial songs. For the most part they are arranged according to a certain recurring order in the deities invoked; and, as we have seen, some poems are included which could never have been sung at sacrifices at all. Besides this collection there are two collections of the liturgic prayers which ought to accompany the performance of sacrifice. The Samaveda comprises the prayers sung at the offering of the soma; they are verses taken from the Rigveda, and the collection is a book of songs or hymns.[191] The Yajur-veda contains the formulæ and ritual which must be chanted at the dedication of the altar, the kindling of the fire, and every act of every sacrifice. Thus the Samaveda supplied the knowledge of the Udgatar, the prayers during the sacrifice of soma, the Yajur-veda supplied the knowledge of the Adhvaryu, who had to perform the material part of the sacrificial service, the ritual for the separate acts of the ceremony. Compared with these two books the Rigveda was the book of the Hotar, _i.e._ of the chief priest, who had to conduct the sacrifice, and invoke the gods to come down to it.[192] If in the parts of the hymns of praise and invitations, which are repeated from the Rigveda in the Samaveda, the style and tone is often more archaic than in the Rigveda, the explanation is that the prayer at the sacrifice was no doubt preserved with more liturgic accuracy, than the invitation to the god, which preceded the sacrifice. The Yajur-veda is preserved in a double form; of which one, the black Yajus, is shown to be the older by its want of systematic sequence; but even in this older form we find, as in the tenth book of the Rigveda,[193] pieces of later origin, the outcome of priestly meditation.