is excluded, and twelve are exposed to the sunlight. At the end of each month one of the exposed pieces is withdrawn from the light, carefully marked with a diamond to show the length of time exposed, and placed in the small dark box. At the end of the year, therefore, the collection embraces fourteen pieces—two of the original color, and twelve showing the effects of exposure from one to twelve months. Other collections may be made to show the daily, weekly, and yearly progress of the sun's rays in changing the color of the glass.
Mr. Gaffield found that, in the time required to produce a change, different specimens of glass presented widely different qualities, the change being much more easily effected in some than in others. In some specimens a marked change of color was observed in a few days; others, after resisting the powerful influence of the solar rays for years, were finally overcome, and made to assume a new color. Several kinds, in which no perceptible change took place in three months, were very sensibly affected by an exposure of a year. But in almost all the change took place.
"It is very interesting," says Mr. Gaffield, "to witness any one of these series of specimens, showing, as in one of white plate, a gradual change, commencing in a day or a few days in summer, from greenish or bluish white, to a yellowish white, or light yellow, a deep and deeper yellow, until it becomes a dark yellow or gold color; and, in some Belgian sheet specimens, a gradual change, commencing in a few weeks in summer, from brownish yellow to deeper yellow, yellowish pink, pink, dark pink, purple, and deep purple."
One interesting experiment was carried on for one year with nine different kinds of glasses, representing plate, crown, and cylinder glass, the manufacture of both hemispheres, and almost every shade and color of what are known as colorless glasses. The results were as follows:
| KIND OF GLASS. | Color before Exposure. | Color after Exposure. |
| French white plate | Bluish white | Yellowish. |
| German crystal plate. | Light green. | Bluish tinge. |
| English plate | "" | Yellowish green. |
| English crown | "" | Light purple. |
| Belgian sheet | Brownish yellow. | Deep" |
| English sheet | Dark green. | Brownish green. |
| American crystal sheet | Light bluish white. | Purplish white. |
| """ | Lighter "" | Light yellowish green. |
| "ordinary. | Bluish green. | No change. |
These colors appear from an observation of the glass edgewise, when is seen a body of color two or four inches in depth, whereas the usual thickness of the glass varies from one-fourteenth to one-quarter of an inch, and shows its color easily only when a white curtain or paper is placed behind it.
Among other experiments made by Mr. Gaffield, two may be noticed as of peculiar interest, and as suggesting a process of producing
