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LXVIII. On the Action of Sunlight on Glass. 

 By Thomas Gaffield*. 



THE great attention now given to all the phenomena con- 

 nected with light and heat may awaken some interest in 

 the experiments in which I have been engaged for the past four 

 years on the subject named at the head of this article. Perhaps 

 I cannot better commence my essay than by quoting, from the 

 c Proceedings of the Nat. Hist. Society of Boston' (vol. ix. p. 347), 

 an account given before that Society of my experiments in 1863, 

 and after I had been engaged in them only a few months. 



" He believed that his experiments in connexion with the 

 subject were original as to their method and their extent, although 

 it had long been observed in Europe that colourless or light- 

 coloured plate glass had turned to a purple hue by exposure to 

 intense sunlight. One case is cited f of a change to a gold colour; 

 and one experiment recorded by Dr. Faraday J, 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 colours as media in the transmission of light and of 

 heat, but none, with 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 colour 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 colour 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 experi- 

 ment with several specimens of plate, crown, and sheet glass 

 during the month of July last, which proved 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 colour 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 colour. 



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

 continued three months on some thirty specimens from France, 

 England, Belgium, Germany, and the United States, only con- 



* From Silliman's American Journal for September and November 1867. 

 t Journal of the Society of Arts for February 15, 1854. 

 X Chemical Researches. London, 1859, p. 142. 



