Mosaic[1] pavements were first introduced in the time of Sylla; at all events, there is still in existence a pavement, formed of small segments, which he ordered to be laid down in the Temple of Fortune, at Præneste. Since his time, these mosaics have left the ground for the arched roofs of houses, and they are now made of glass. This, however, is but a recent invention; for there can be no doubt that, when Agrippa ordered the earthenware walls of the hot baths, in the Thermæ which he was building at Rome, to be painted in encaustic, and had the other parts coated with pargetting, he would have had the arches decorated with mosaics in glass, if the use of them had been known; or, at all events, if from the walls of the Theatre of Scaurus, where it figured, as already[2] stated, glass had by that time come to be used for the arched roofs of apartments. It will be as well, therefore, to give some account, also, of glass.