ACTION OF SUNLIGHT ON GLASS. 



69 



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 each month one of the exposed pieces is withdrawn from the light, 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. Sev- 

 eral 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 : 



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 no- 

 ticed as of peculiar interest, and as suggesting a process of producing 



