In Rome, however, the art of glass manufacture found a congenial home and was developed to a high pitch of excellence. So widespread was its use that it is a truism to say that in Rome of two thousand years ago glass was employed for a greater number of purposes--domestic, architectural, and ornamental--than it is to-day, even though the glazing of windows was in its infancy and the use of the material for optical purposes was scarcely known. In effect, coloured and ornamental glass held much the same place in the Roman household that china and earthenware do among us to-day. Glass was used for pavements and for the external covering of walls. The Roman glass-workers were particularly happy in their combination of colours, both by fusing together threads of various colours, or by fusing masses, so as to imitate onyx, porphyry, serpentine, and other ornamental stones. The most interesting of all was the famous cameo glass. A bubble of opaque white glass was blown, and this was coated with blue and a further layer of opaque white superimposed. The outer coat of blue was removed from the portion which was to display the design, leaving the white to be carved into whatever figures the artist’s fancy dictated. The finest example extant of this kind of ware is the famous Portland vase in the British Museum. The art, thus brought to such perfection in Rome, naturally spread throughout Italy and the Roman colonies in France, Spain, Germany, and Britain. Probably workmen from the Italian cities also established the first furnaces among the lagoons of Venice, and so laid the foundation of what were to be the finest glass manufactories in the world. At the end of the thirteenth century a guild of glass-workers was formed. These sequestered their craft upon the island of Murano, and there cultivated it with an increasing skill that in a brief space made Venetian glass the marvel of the civilised world. The peculiar merits of the Venetian product were grace of form and lightness of execution. Many of the vessels are surpassingly thin. The quality of the metal, however, leaves something to be desired. It is dull, frequently tinged with yellow--due to the presence of iron--or purple--the effect of too great a proportion of manganese. The workmen became so skilful that, carried away by the _joie d’exécuter_, they produced not only the artistic forms for which Venetian glass is famous, but all sorts of extravagances--ships, animals, birds, fishes, and so on--whose only merit was to testify to the excellence of a technique which could so triumph over the difficulties of form and material. Meanwhile, other European nations had taken their cue from Venice, and glass-houses sprang up in various parts of the Continent, particularly in France and in Bohemia; the latter, indeed, speedily became the great rival of Venice.