the Colouring of Glass hy Metallic Oxides. 443 



made is placed in a muffle too mucli fired, the bright orange- 

 red colour turns first to crimson-red^ then to purple; by a 

 greater heat it takes a bluish tinge, and afterwards gets dis- 

 coloured ; it is therefore acknowledged that ruby glass must 

 be exposed to the lowest temperature possible to obtain the 

 brightest tints. From these observations we conclude that 

 glass in which copper is kept in the state of protoxide by 

 addition of tin or carbonaceous matters, is apt to acquire suc- 

 cessively all the colours of the spectrum, under circumstances 

 which do not appear to be the effect of modification by oxygen. 



4. Silver. 



Oxide of silver is seldom added to the mixtures which are 

 to be melted in glass furnaces, but is generally used to stain 

 glass of a transparent yellow, on the surface of which it is laid 

 and fired. This colour is produced without any addition of 

 fuM ; it is only necessary to lay on the surface of the glass or 

 flint-glass a small proportion of oxide, or any salt of silver in 

 a great state of division, mixed with a neutral medium, such 

 as pounded clay or red oxide of iron, and to expose this glass 

 to the heat of a muffle; the medium is afterwards taken off by 

 brushing the surface of the glass, and the glass is stained of a 

 yellow colour, which varies between lemon or greenish-yelloiio 

 and dark orange, according to the quantity of silver, and 

 especially to the quality of the glass ; a red colour can even be 

 produced by exposing the glass twice to the heat of the muffle. 

 The celebrated Dumas has found by accurate analysis, that 

 the glass which was liable to take the deep tints had its ele- 

 ments the nearest in definite proportions ; which agrees with 

 this observation, that the glass must have been deprived of all 

 excess of alkali by a long melting at a high temperature, to 

 take the deep tints of orange and red. 



It is important not to heat the muffle to too high a tempe- 

 rature, otherwise the surface of the glass on which the silver 

 has been laid becomes opalescent, although when seen through 

 it still remains yellow or orange: the glass viewed obliquely 

 reflects an opake blue colour, and at a still higher temperature 

 it is liable to appear of a pink colour when seen through, 

 although the opacity of the surface is still increased, and 

 becomes brownish-yellow. 



If, instead of staining the glass in a muffle, silver added to 

 a mixture of flint-glass is melted in covered pots in the short- 

 est time possible, the result is an agatized semi-opake matter, 

 which, by the combined effects of refraction and reflexion, will 

 present all the colours of the spectnwi ; this effect is most sen- 

 sible, if the surface of the glass, which is generally yellowish- 



