70 THE POPULAR SCIENCE MONTHLY, 



very delicate designs and pictorial effects on glass. In the first, an 

 inscription was made on a piece of Belgian sheet-glass, in part with 

 gold and silver leaf, and in part witli black and white paint. The 

 gold and silver leaf were washed off, but the letters painted in black 

 and white remained. After an exposure of nearly two years the surface 

 of the glass was cleaned, when the clearly-marked words of the inscrip- 

 tion appeared in the original color of the glass, while the surrounding 

 portions were changed by the action of the sunlight to a purple color. 

 By the second experiment the gradually-increasing effect of sunliglit 

 on glass may be shown by exposing to the solar rays a piece of easily- 

 changed glass. Take a piece about twenty inches long by four wide, 

 and at each end cover a strip about four by two inches with black 

 paint. At the end of one month, two months, and at biennial periods 

 thereafter, paint an additional similar strip in black, until the entire 

 piece is painted. Then, upon removing the paint, there will appear a 

 single piece of glass presenting the original color, and all the grada- 

 tions of color and hue presented by exposure from one to thirty-six 

 months. 



In 1825 Faraday thought that only glass containing oxide of man- 

 ganese was subject to this change of color. In 1867 M. Pelouze did 

 not " believe that there exists in commerce a single species of glass 

 that does not change its shade in the sunlight." The results of Mr. 

 Gaffield's experiments have led him to " affirm that a longer or shorter 

 exposure to the direct action of the sun's rays will probably change in 

 some degree the color of all or nearly all kinds of window-glass," and 

 that the phenomenon is not limited to glass containing oxide of man- 

 ganese. It should be observed that Mr. Gaffield's statement is limited 

 to the ordinary window-glass, although embracing many different 

 kinds of that class. 



Specimens of flint and of colored glass have also been subjected to 

 the test, but, with one or two exceptions, without exhibiting a change 

 of color. An experiment, continued for five years, with red, yellow, 

 green, blue, and purple pot-metal, i. e., glass colored in the pot, pro- 

 duced no change in any case except the purple. Still, this does not 

 prove that changes may not be effected by longer exposure. Subse- 

 quently, Mr. Gaffield experimented with pot-metals, not of the primary 

 colors, but of the intermediate ones, which most nearly approach those 

 produced in colorless glass by sunlight exposure. In every specimen 

 of the brownish, yellowish, and rose or purple colors thus exposed, a 

 change in color or shade was produced in a short time. A change 

 was also observed in the colorless body of some of the specimens of 

 flashed and stained glass. 



As pot-metal colors of this class were used in the early-painted 

 windows, it is pertinently asked whether these experiments may not 

 throw some light upon the many interesting questions relating to the 

 alleged superiority of the old cathedral-glass. 



