Mr. T. Gaffield on the Action of Sunlight on Glass. 515 



firmed the results of the first ; and a daily examination at first, 

 and afterwards from week to week and month to month, revealed 

 the interesting fact that even after a single day's exposure to a 

 July sun the change of colour will, in some instances of the 

 lightest hues, commence. 



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

 all the light-coloured glasses, that he commenced a third expe- 

 riment on the 6th of August which should speak for itself. He 

 then exhibited to the Society ten pieces of French white plate 

 glass, 4 by 2 inches in size (all of which were cut from the same 

 sheet), one of which showed the original colourless 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. 



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

 specimens were so yellow as to exhibit a contrast very marked, 

 and excited the interest of all the members present. That the 

 colour permeates the body of the glass and is not confined to the 

 surface, or produced by reflexion therefrom, has been conclu- 

 sively proved by grinding 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 yel- 

 low colour. 



" The glasses exposed were all what are called colourless win- 

 dow-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 coloured 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 corre- 

 sponding to those exposed, and cut off from the same pieces, 

 were placed in the dark to be compared with the other speci- 

 mens after exposure. 



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

 did not change. The colour 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 greenish colour, made in Germany, 

 and at the only factory which has not given up the use of potash 

 for soda-ash. 



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

 might change every colour in some degree. 



