440 M. G. Bontemps on some modifications in 



able to explain the observed phaenomena. In more modern 

 times, by the aid of chemical science, we have been able to 

 analyse the metallic oxides, and their various combinations 

 with acids. By analogy, glass having been considered as a 

 salt with simple or multiple bases, general axio7ns were ad- 

 mitted in the colouring of glass by metallic oxides. It is said, 

 for instance, that the silicates oi potash and of soda are colour- 

 less; the silicate of^ potash or soda and 7nanga7iese IS purple ; the 

 silicate of potash or soda and cobalt is bhie; the silicate of pot- 

 ash and deutoxide of copper is blue; the silicate of potash and 

 protoxide of copper is rcd\ the silicate of potash and gold is 

 pink^ &c. Such axioms are quite sufficient for those who 

 want only a superficial knowledge ; but in entering more 

 deeply into the investigation of the phaenomena produced by 

 the use of metallic oxides in glass-making, it will soon be 

 acknowledged how fertile is the field of observations, and how 

 incomplete is their explanation. 



Allow me to mention some of the phaenomena produced by 

 a few metals; several of them will perhaps have for many 

 persons the charm of new facts, although these metals are 

 those most generally used for colouring glass. 



1. Iron. 



It is generally admitted that oxide of iron gives a greenish 

 colour to glass to the mixture of which it has been added ; 

 but the truth is, that this colour is produced only in peculiar 

 circumstances. 



The manufacturers of china, porcelain and earthenware, 

 are well-aware that oxide of iron is the colouring material of 

 a fine purplish-red enamel fired in their muffle (and it is quite 

 clear that enamels are real glass) ; if the temperature were 

 raised too high, this enamel would lose its purplish tinge and 

 tend towards orange ; so that three colours of the spectrum 

 are produced by oxide of iron, even at degrees of heat which I 

 should call low, compared with the temperature of furnaces 

 for glass melting, which we shall now consider. 



If into a pot containing white melted glass or flint-glass we 

 introduce during the blowing a small fragment of iron, it 

 will, from its gravity, fall to the bottom ; now, if after the 

 blowing, this pot is taken out of the furnace, we shall see close 

 to the fragment of iron partly oxidized, a portion of the glass 

 coloured from orange to yellow. We have also an illustration 

 of the j/e//oto colour produced by oxide of iron in the manu- 

 facture of artificial aventurine. It is known that this aventu- 

 rine is produced by the exposure of soft glass containing a 

 large proportion of the oxides of copper and iron, to a tempera- 



