348 



He believed that his experiments in connection with the subject 

 were original as to their method and their extent, although it had long 

 been observed in Europe that colorless or light-colored plate-glass had 

 turned to a purple hue by exposure to intense sunlight. One case * 

 is cited of a change to a gold color ; and one experiment recorded by 

 Dr. Faraday, f some forty years ago, proving that a light purple 

 changed to a darker hue after eight months' exposure. 



Other experiments are on record showing the action of glass of 

 different colors as media in the transmission of light and of heat ; but 

 none, Avith the above exception, showing the effect produced on the 

 glass itself 



An experience of some twenty years in the window-glass business 

 had only presented a few isolated cases of supposed change of color 

 from this cause, which were attributed to some obvious defect in an 

 article of inferior manufacture ; but, within a short time, he had 

 heard of the change of color in an article of superior manufacture, in 

 a quantity of white plate-glass,, of which some lights had been broken 

 out of a window in which they had been exposed to the sun. 



This fact coming to his knowledge led him to try an experiment 

 with several specimens of plate, crown, and sheet glass, during the 

 month of July last ; which pi-oved that a month's exposure to a hot 

 sun would change the best white French plate and all white sheet 

 glass, such as is used for photographs and engravings, to a color 

 containing more or less of a yellow hue. The dark green and dark 

 blue or bluish green did not experience any change ; but any hue 

 which approached a white, whether bluish, greenish, or yellowish 

 white, turned to a yellowish color. 



A second series of experiments, commenced in July, and continued 

 three months, on some thirty specimens from France, England, Bel- 

 gium, Germany, and the United States, only confirmed the results of 

 the first ; and a daily examination at first, and afterward from week 

 to week, and month to month, revealed the Interesting fact, that, 

 even after a single day's exposui'e to a July sun, the change of color 

 will, in some Instances of the lightest hues, commence. 



So remarkable was the change in a week, affecting nearly all the 

 light-colored glasses, that Mr. GafBeld commenced a third experiment 

 on the 6th of August which should speak for itself He then exhibited 

 to the Society ten pieces of French white plate-glass, four by two Inches 

 in size (all of which were cut from the same sheet), one of which showed 

 the original colorless glass, and the others exhibiting the change of 

 hue towards yellow, after exposure respectively of one, two, and four 

 days ; one, two, and three weeks ; one, two, and three months. 



* Journal of Society of Arts for Feb. 15, 1854. 



i Dr. Faraday's Chemical Researches. London, 1859, p. 142. 



