524 Mr. T. Gaffield on the Action of Sunlight on Glass. 



the specimens offered him for sale. And all manufacturers can 

 make them practical by making their glass of pure materials, 

 which will not have to be " doctored," to use the glass-makers' 

 term for the use of manganese ; or by allowing the glass to as- 

 sume its natural colour, even if it be a little blue or green, 

 rather than to run the risk of its subsequent change to yellow 

 or purple by exposure to sunlight. 



In the Comptes Rendus for January 14, 1867, Pelouze says 

 (and we believe he is the first and only writer who has made 

 this observation) : — 



"Exposure to red heat decolorizes the glasses which have 

 been made yellow by sunlight ; or, to speak more exactly, they 

 retake the light-green shade which they had before exposure. 

 A second exposure to sunlight produces a second coloration 

 similar to the first; and a red heat makes it disappear again. 

 These phenomena can be reproduced indefinitely. The glass 

 preserves its transparency and does not give place to any striae 

 or bubbles."* He also says : — 



"I possess specimens of glass rendered violet by sunlight. 

 AH present the property of being decolorized by heat. A tem- 

 perature of 350° is not sufficient. It is necessary to have that 

 employed in the reheating of glass in general; and that is in the 

 vicinity of red heat. The glass decolorized by heat, when ex- 

 posed to sunlight, retakes the amethyst colour which it acquired 

 the first time, loses it anew when it is heated; and these curious 

 phenomena can be reproduced without cessation." 



In confirmation of this most interesting statement of Pelouze, 

 I have exposed in a glass-stainer^s kiln several specimens of 

 glass which had been changed by the action of sunlight, some 

 to a yellow and some to a purple colour. The exposure to an ex- 

 treme red heat made the glass assume, some a white, some a 

 yellowish white, and some a green colour, which were probably 

 the original colours. These specimens were taken from win- 

 dows where they had been exposed from a few years to more 

 than half a century. Further experiments, which I have already 

 commenced, will show whether we can reproduce the exact ori- 

 ginal colours by heat after they have been changed by exposure 

 to sunlight. 



We have in the same kiln exposed some dozen original and 

 unexposed specimens of what are called colourless window- 

 glasses of different kinds and shades of colour, and found them 

 unchanged in the slightest degree by the action of great heat, 

 while similar specimens have been changed in a few days, weeks, 

 or months by the simple action of the sun's rays. Fifteen speci- 

 mens of really coloured glasses (red, green, yellow, &c.) have 

 been exposed in the same way without any change of colour, 



