Other Greek treatises of a similar character, which are either lost or survive only in extracts or translations, are attributed to Zosimus, a writer of the third century, who had a section on glass; to Synesius, a Cyrenaic bishop (400 A.D.), married, and half a pagan; and to Olympiodorus, a priest of Isis, who in the sixth century kept up some of the Hellenic traditions. The late Byzantine scholiasts drew up summaries of these treatises and of many others; an important manuscript of this class at Venice gives a list of fifty-two such works. But these Byzantine summaries are of little value to us; all grip of fact is completely lost in the mystical jargon of the school. Of much greater interest are the many series of practical formulas written in Latin, beginning with the _Compositiones ad Tingenda_, known to us from a manuscript of the eighth century. Here we find a section upon the colouring of artificial stones, their gilding and polishing, and upon the colouring of glass generally—how it is rendered milky by means of tin, red by cinnabar (?), by litharge (?), and by a substance called _calcocecaumenon_, the latter word doubtless a corruption of the Greek equivalent of the _æs æstum_, or burnt bronze, the well-known mediæval source of an opaque red. Further on recipes are given for other colours to be applied as varnishes. There is also a chapter on the making of glass and some summary account of glass-furnaces, interesting solely as the earliest example of the many such descriptions that have come down to us. In fact, all these writers copied one from the other, summarising or amplifying. The same recipes, more or less intelligently expressed, turn up again and again: we can trace them in Theophilus, and even in such comparatively modern writers as Neri and Kunckel. A later treatise, the _Mappæ clavicula_ (ninth or tenth century), follows closely upon the _Compositiones_. As regards glass, we find headings—for that is unfortunately all that remains of this section—on unbreakable glass, on the soldering of glass, on the art of tracing trees and fruits of all kinds upon a flask, on an indelible manner of painting on glass, and finally, three sections on the fabrication of pearls. I have already discussed one of the recipes of Heraclius (or Eraclius) when describing the cemetery glasses. All that we know of this writer is that he was a monk, and that he probably wrote in Rome, not later than the tenth century. The twenty-one little sections that make up his two books are written in hexameters, and treat of _The Colours and Arts of the Romans_. A third and much larger book in prose, that is found in some manuscripts, is of a considerably later date and of quite a different nature.[84] I will now briefly summarise what the true Heraclius has to say about glass in his two metrical books.