For some account of what these writers tell us about glass, see below, Chap. VII. Footnote 45: Theophilus, however, writing a century earlier than the pseudo-Heraclius, appears to speak of the marver as a slab of stone (see below, Chap. VII.). Footnote 46: The sand of this river as a material for the manufacture of glass is already mentioned by Theophrastus, a pupil of Aristotle. Footnote 47: _Glebas nitri._ This is doubtless the natron (impure carbonate of soda) exported from the Egyptian natron lakes, which have been worked from a very early period—a substance that must not be confused with our nitre (nitrate of potash); as I have said, the glass of the ancients is essentially a soda glass. The natron was probably first exported for the use of the soap-makers. Footnote 48: This again must not be confused with the white earth, which we now know under that name, a substance unknown to the ancients. Footnote 49: By this is probably meant three parts in twelve or ten, _i.e._ 25 or 30 per cent. of the whole. Footnote 50: Great care must be exercised in translating the names of the precious stones and marbles mentioned by Greek and Roman writers. These names are used in the vaguest way, which hardly ever corresponds to the modern meaning. Footnote 51: Among others, from the early history of the Christian Church in these parts. Footnote 52: At Rome, too, there is some reason to think that the working of glass—the minor departments of that art, at least—was long in the hands of Syrian or other Semitic immigrants. Martial’s itinerant hawker from the Transtevere, who bartered his sulphur matches for broken glass, we may perhaps think of as a Jew (Book 1., _Epigram._ 42). Footnote 53: See p. 88. Footnote 54: Compare with these the bottle from Cologne in the British Museum containing a hardened mass of some yellow substance, and closed by a decayed cork partly covered by a corroded bronze capsule (_Slade Catalogue_, No. 275). Footnote 55: Both these forms are found in Anglo-Saxon and Frankish graves. It will be remembered that in France there was no sudden break in the Roman culture on the appearance of the Germanic invaders, as was the case in England. Footnote 56: Philostratus describes the process by which the ‘barbarians of the ocean’ spread colours upon heated bronze so as to form a hard enduring decoration. He was of the household of Julia Domna, and M. Froehner suggests that he may have heard of these enamels from one of the officers of the army of Septimius Severus. Footnote 57: The famous enamelled bowl, however, found in a Roman tomb of the time of Hadrian, at Bartlow, Essex, was accompanied by a cinerary vase and other examples of glass. See _Archæologia_, vol. xxvi. Footnote 58: