Page:Popular Science Monthly Volume 5.djvu/78

68 Berlin: "I would mention a curious fact, in which the sunbeams have, if I may say so, done something in the art of penmanship—not only on the surface, but by inscribing characters through the body of the glass; and though the matter is based upon causes well known by experience, yet there has probably never before been so striking an instance of their effect known. I am in possession of a plate of glass which was used as a window-pane for more than twenty years, and on which was an inscription in gold letters. This inscription was taken off by grinding the plate on both sides, and polishing it so as to have a new surface. When the glass had been polished, the inscription could again be clearly seen. The parts which had been under the letters remained white, while the remainder of the plate had assumed a violet tint, in consequence of the manganese it contained, a coloring which permeates the whole mass, as the grinding of the surface proved. The uncovered part of the plate, especially when laid upon a white background, shows the clearly-readable characters."

From the above, it will be seen that the power of the sun's rays to change the color of glass has been publicly announced for at least a half-century; but it does not appear that elaborate and systematic experiments upon this subject were instituted prior to those referred to in the opening sentence of this article. These were begun in 1863, by Mr. Thomas Gaffield, a window-glass merchant of Boston, who has made an enthusiastic study of many matters pertaining to glass, and whose collection of authorities on this and kindred subjects is probably not equaled by any private, and by very few public, collections in existence. These experiments now cover a period of eleven years, and embrace some eighty different kinds of glass, of English, French, German, Belgian, and American manufacture, including specimens of rough and polished plate, crown, and sheet window-glass; flint and crown optical glass; opal and ground glass; colored pot-metal (i. e., glass colored in the pot during the process of melting); flashed and stained glass of various colors; and glass-ware and glass in the rough metal. The experiments have been conducted with pieces of glass usually four by two inches, of which several hundred specimens have been exposed, showing the effect of sunlight in producing a change of color by exposure, from one day in summer to several years. The changes produced in the colorless glasses are from white to yellow, from greenish to yellowish green, from brownish yellow to purple, from greenish white to bluish white, and from bluish white to a darker blue.

Mr. Gaffield's plan of procedure has been to cut a number of pieces of the size mentioned above, from the same sheet of glass, the number depending upon the nature of the experiment to be made. Suppose that white plate-glass is to be tested by exposure from one to twelve months: fourteen pieces, precisely alike, are cut from the same plate; two are carefully put away in a neat box, from which the light