If these bones be touched they will generally crumble into a white, flaky powder. D’Agincourt copied a tomb (Fig. 7) in which this “dry dust of death” still retained the outline of a human skeleton. Verily, “_Pulvis et umbra sumus._” Sometimes, however, possibly from some constitutional peculiarity, the bones remain quite firm notwithstanding the lapse of so many centuries. De Rossi states that he has assisted 24 at the removal of a body from the Catacombs to a church two miles distant without the displacement of a single bone.[12] The age of the deceased and the nature of the ground also affect the condition in which the remains are found. Of the bodies of children nothing but dust remains. Where the _pozzolana_ is damp, the bones are often well preserved; and where water has infiltrated, a partial petrifaction sometimes occurs.[13] Campana describes the opening of a hermetically sealed sarcophagus, which revealed the undisturbed body clad in funeral robes, and wearing the ornaments of life; but while he gazed it suddenly dissolved to dust before his eyes. Sometimes the sarcophagus was placed behind a perforated slab of marble, as shown in the following example, given by Maitland. The lower part of the slab is broken. [Illustration: Fig. 7.--Valeria Sleeps in Peace.] [Illustration: Fig. 8.--Arcosolium with Perforated Slab.] The other essential constituent of the Catacombs, besides the galleries already described, consists of the _cubicula_.[14] These are chambers excavated in the _tufa_ on either side of the galleries, with 25 which they communicate by doors, as seen in Fig. 4. These often bear the character of family vaults, and are lined with graves, like the corridors without. They are generally square or rectangular, but sometimes octagonal or circular. They were probably used as mortuary chapels, for the celebration of funeral service, and for the administration of the eucharist near the tombs of the martyrs on the anniversaries of their death. They were too small to be used for regular worship, except perhaps in time of persecution. They are often not more than eight or ten feet square. Even the so-called “Papal Crypt,” a chamber of peculiar sanctity, is only eleven by fourteen feet; and that of St. Cecilia adjoining it, one of a large size, is 26 less than twenty feet square. Even the largest would not accommodate more than a few dozen persons. These chambers are generally facing one another on opposite sides of a gallery, as in the annexed plan of two _cubicula_ in the Catacomb of Callixtus. [Illustration: Fig. 9.--Plan of Double Chamber.]