349 



The changes in the first four days were slight ; but the last spe- 

 cimens were so yellow as to exhibit a contrast very marked, and 

 excited the interest of all the members present. That the color perme- 

 ates the body of the glass, and is not confined to the surface, or pro- 

 duced by reflection therefrom, has been conclusively proved by grind- 

 ing off about one-sixteenth of an inch from both surfaces and the 

 four edges of a duplicate exposed specimen, which, after repolishing, 

 still exhibited the same yellow color. 



The glasses exposed were all what are called colorless window- 

 glasses, although they varied in tinge and hue from the whitest French 

 plate to the darkest green English sheet-glass. ' 



An experiment for four months, from July to November, on really 

 colored glasses, red, green, yellow, blue, and purple, showed no change 

 except in the purple, which became slightly darker. 



The experiments were carried on upon a rough plate-glass roof, 

 nearly horizontal, and which received the rays of the sun during the 

 greater part of the day. In all cases, strips corresponding to those 

 exposed, and cut off" from the same pieces, were placed in the dark, 

 to be compared with the other specimens after exposure. 



It will be noticed that the dark green, blue, and bluish green did 

 not change. The color of the Belgian sheet (called German or French 

 by glass-dealers in America), a yellowish or brownish green, did not 

 change ; and these were the only exceptions. All plate-glasses changed, 

 except an inferior blue quality, and a superior crystal plate of a green- 

 ish color, made in Germany, and at the only factory which has not 

 given up the use of pot-ash for soda-ash. 



It is possible that a longer exposure of a year, or of years, might 

 change every color in some degree. 



His inquiries, since he instituted these experiments, have brought 

 out some fine specimens of Belgian sheet-glass from a house built 

 three years ago, which had changed in some instances to a golden, 

 and in others to the well-known purple hue. 



It is Mr. Gaffield's intention to pursue the experiments farther, with a 

 view to ascertain the effects of sunlight during each month and season 

 of the year; and also whether exposure to heat, air, or moisture 

 alone, out of the direct action of the sun's rays, will produce any cor- 

 responding change. 



Mr. Gaffield does not propound any theory to explain these changes 

 of color, which, under our sunny skies, pi-obably take place much more 

 rapidly than in the diff"erent and less clear atmosphere of England. 



Some writers point to the presence of oxide of manganese in the 

 original composition of window-glass, and some to the oxyde of iron, 

 as a chief cause. 



Some writers have peculiar theories abqut the different classes of 



