ANCIENT AND MEDIAEVAL CHEMISTRY. 117 



The title of one of the recipes in the old table, " How to make 

 unbreakable glass," deserves to be dwelt upon, on account of the 

 legends and traditions that are associated with it, and which have 

 been perpetuated down to our own time. Unbreakable glass ap- 

 pears to have been really discovered under Tiberius, and gave rise 

 to a legend according to which its properties were amplified and 

 it was made malleable. Tiberius, according to Pliny, caused 

 the factory to be destroyed, for fear that the invention would 

 diminish the value of gold and silver." " If it was known," wrote 

 Petronius, " gold would become as cheap as mud." According to 

 Dion Cassius, Tiberius slew the author. Petronius, who is re- 

 peated by other authors, says that he was decapitated, and adds 

 that " if vessels of glass were not fragile they would be preferable 

 to vessels of gold and silver." 



These stories relate evidently to the same historical fact, re- 

 ported by contemporaries, but disfigured by legend ; the invention 

 was probably suppressed for fear of its economical consequences. 

 It is very curious to find it mentioned in the goldsmiths' recipes 

 of the middle ages, as if the secret tradition had been preserved 

 in the shops. Some of them claimed that glass could be made 

 malleable and ductile and changed into a metal. A process for 

 making glass that will not break has been discovered in our own 

 times, and is announced unequivocally and in definite shape. In 

 truth, malleable glass was not really in question ; but even that is 

 not a chimera. Industrial processes for beating and molding glass, 

 based on the plasticity and malleability which it possesses at a 

 temperature near fusion, have been described in late years. An 

 article in the Key to Painting seems to point to a similar process. 

 Real properties, perceived doubtless from antiquity and preserved 

 as shop secrets, gave rise to the legend. 



The collection bearing the name of Eraclius or Heraclius is in 

 two parts, of different composition and date. The first part con- 

 sists of two books in verse, having the character of the writing of 

 the end of the Carlovingian epoch, or of the ninth and tenth cen- 

 turies. It treats of vegetable colors, of gold leaf, of writing in 

 letters of gold, of gilding, of painting on glass, and of the prepara- 

 tion of precious stones. All the recipes are of ancient origin, a 

 little vague, and without novelty. A book in prose is more com- 

 pact and precise. It was added later by a continuator, toward the 

 twelfth century, for there is a discussion in it of the coloring of 

 Cordovan leather, and cinnabar, which is red, is called in it azure 

 a translation of an Arabic word, frequent in the twelfth centu- 

 ry, which has given rise to all sorts of misconceptions and con- 

 fusion with our modern azure blue. It has the stories about mal- 

 leable glass ; and most of the subjects were already treated in the 

 Key to Painting. 



