68 THE POPULAR SCIENCE MONTHLY, 



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 ex- 

 perience, 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, collec- 

 tions 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 speci- 

 mens 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 pro- 

 ducing 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 num- 

 ber depending upon the nature of the experiment to be made. Sup- 

 pose 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 



