In his poems Dante is faithful to his own descriptions of the universe in prose, and in the visionary journey of the _Divine Comedy_ he passes successively through all the spheres which are enumerated in the _Convivio_ and the _Quæstio_. The central sphere of earth, the lowest of all the elements, “il suggetto dei vostri elementi,”[643] is completely traversed, from side to side, and on the further side is seen the vast sphere of ocean which envelopes three-quarters of the earth. The Mountain which the poet has placed here (his only innovation) rises out of the water towards the heavens higher than any other, “inverso il ciel più alto si dislaga,”[644] and in climbing it he reaches that region in the sphere of air into which no cold exhalations can rise from earth or sea, neither the “secco vapor”[645] which causes wind and earthquake, nor the “esalazion dell’ aqua”[646] from which are formed rain and hail, cloud and dew. Hot exhalations ascend to the sphere of fire, there to become meteors or comets (see p. 103); but the cold vapours rising as high as they can towards the sun which draws them up (“retro il calor”[647]), are unable to ascend beyond the Gate of Purgatory.[648] Therefore Purgatory is exempt from all changes except those caused by that which heaven receives from itself into itself. This was ingeniously conjectured by Venturi to be light, since there is change of day and night in Purgatory; but from the context it seems clear that human souls are meant. Souls come forth from their Creator, whose special abode is the heaven of heavens, and return thither after purification. And it was the rising of the purified soul of Statius, ready to return to heaven, which had been the cause of that trembling of the Mountain of which he is here speaking. ------------------------------------------------------------------------ [643] “The foundation of your elements.” _Par._ xxix. 51. [644] “Rises highest towards heaven out of the sea.” _Purg._ iii. 15. [645] “Dry vapour.” [646] “Exhalations from the water.” [647] “Behind [_i.e._ following] the heat.” [648] _Purg._ xxviii. 97-99 and xxi. 52-53. The gate is meant by “i tre gradi” (“the three steps”), and “ove si serra” (“where it is locked”). See _Purg._ ix. 76, 106-108. ------------------------------------------------------------------------