318 T. Gaffield on the action of Sunlight on Glass. 
Any photographer can make these observations practical, by 
testing the action of sunlight for six months ora year, on a 
the specimens offered him for sale. And all manufacturers 
can make them practical, by making their glass of pure mate- 
rials, which will not have to be “doctored,” to use the glass- 
makers’ term for the use of manganese; or by allowing the glass 
to assume its natural color, 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 14th, 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 strie 
“T possess specimens. of glass rendered violet by sunlight. 
All present the property of being decolorized by heat. A tem- 
om a few years to more than 
half a century. Farther ex riments, which I have already 
commenced, will show whethe 
it en colors by heat, after being changed by exposure to su 
