Page:Popular Science Monthly Volume 5.djvu/80

70 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 with 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 inscription 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 sunlight 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 gradations of color and hue presented by exposure from one to thirty-six months.

In 1825 Faraday thought that only glass containing oxide of manganese 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 manganese. 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, produced no change in any case except the purple. Still, this does not prove that changes may not be effected by longer exposure. Subsequently, 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.