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



bably very little, if any, manganese, changed not in two years. 

 Perhaps a longer exposure may produce some change. 



A rough piece of light-coloured window-glass metal changed 

 to a yellowish colour in a year. 



Coloured glasses after two or three years* exposure showed no 

 perceptible change in any instance, except a slight one in a single 

 specimen of purple. Perhaps an exposure of many years may 

 make a change in some other colours. 



I have made experiments with artificial heat on glass in various 

 ways, from exposure to the heat of a cooking-range oven to that 

 of a glass-stainer's kiln, without any change of colour in the 

 common colourless window-glasses, while the same or similar 

 specimens exposed to sunlight have been nearly all changed in a 

 few months. 



Specimens exposed in hot water for a month, indoors and out 

 of sunlight, experienced no change of colour; while similar ones 

 exposed during the same length of time in the bottom of a dish 

 filled with two or three inches of water out of doors, and to the 

 direct rays of the sun, experienced a decided change, though 

 only about half as much as when directly exposed out of the 

 water. 



Being convinced that air, moisture, and artificial heat do not 

 cause any change of colour, our experiments indicate that the 

 change is effected by the actinic rays of the sun alone. 



This actinic effect is cut off in some degree by every medium, 

 by water as stated above, and even by clear glass, as a specimen 

 exposed inside of a window or under another piece of thin colour- 

 less glass shows only about one-half as much change as that ex- 

 posed outside of the window or with no covering of glass over 

 it. The amount cut off by colourless glass and by coloured 

 glass differs greatly with the difference of colour. 



The comparative power of glass of different kinds to transmit 

 the actinic rays I have tested by placing underneath pieces of 

 each kind, pieces of easily changing glass (white plate or Belgian 

 sheet glass), exposing them one year, and noticing at the end of 

 that period the comparative depth of the yellow or pink colour 

 to which the under pieces had changed. The result of my ex- 

 periments proved that the most easily transmissive of the colour- 

 less glasses were the English crown, French plate, two kinds of 

 white crystal sheet made in Massachusetts (from the celebrated 

 Berkshire white sand), the New Jersey sheet glass, one kind of 

 English plate, and one kind of Belgian sheet, and about in the 

 order in which I have named them. 



Of the coloured glasses, the blue transmitted the most, the 

 purple less, the red and orange the least — the glasses under 

 these two and the yellow and green showing little or no change. 



