322 T. Gaffield on the action of Sunlight on Glass, 
rays, which have here been noticed. ; 
Theories.—The interesting phenomena of which I have given 
glass ware, materials are used which are not perfectly and 
chemically pure. The sand, the carbonate or sulphate of soda, 
and the lime, one or all, contain slight impurities of iron, the 
protoxyd of which gives glass a green color. To correct this, 
the batch is 
3 g ve 
: Foes writing the above, by the kindness of Mr. John A. Whipple, 
ewan photographer of Boston, n enab 
darkening of sensitive paper under 
eHect of exposure to sunlight for one 
