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



I have made an experiment for one year with two kinds of 

 easily changing glass out of doors, and out of the direct rays of 

 the sun, and found that they were both slightly affected and 

 changed towards a yellowish colour. I did not expect any change, 

 but can perhaps properly account for it on the ground that it 

 was the result of the action of diffused sunlight. It is barely 

 possible that the sun may for a few minutes in some days of the 

 year have cast some reflexions when I was not present in the 

 dark corner in which I placed my specimens. 



It may be that the action of the sun's heat produced the 

 slight effect noticed. If so, it would be an interesting confir- 

 mation of TyndalFs experiments, and of his theory of the cor- 

 relation of forces. I do not consider my single experiment 

 entirely conclusive, and shall make others which will give us 

 more material for proper theories and conclusions. 



The experiments which I have carried on for four years em- 

 brace one specimen of optical glass, a few kinds of flint glass, 

 and glass ware, sixteen kinds of French, Belgian, German, and 

 English plate glass, four kinds of American, English, French, 

 and Belgian rough plate, two of American and English crown 

 glass, ten kinds of American, Belgian, French, and English 

 white sheet glass, four kinds of American, Belgian, and En- 

 glish ordinary sheet glass, fifteen kinds and shades of English 

 coloured glass, four of opake, white-enamelled, and ground glass, 

 and one piece of the rough metal of American sheet glass, — in 

 all, about sixty varieties. 



I have watched and recorded in some experiments the results 

 from day to day, in others from month to month and season to 

 season. 1 have now commenced a series in which I may record 

 results from year to year for ten years or more. In these it 

 may be found that specimens of what are called colourless glasses 

 changed to a yellow colour by exposure for a year, may by much 

 longer exposure be turned to a yellowish pink and a purple. 

 And some which have been entirely unaffected may be affected 

 by an exposure for ten or twenty years. Perhaps some of the 

 coloured glasses may show signs of a change of hue or shade. 



These new experiments include rough and polished plate, 

 crown, cylinder, ground, enamelled, and coloured glass. I have 

 also begun to expose under several of these kinds of glass pieces 

 of easily changing glass, which I shall take in from year to 

 year, these under pieces showing the power of these glasses above 

 them to transmit the actinic rays. 



The most easily changing glasses are a certain kind of white 

 plate, which changes from a white to a yellowish colour, and a 

 certain kind of Belgian sheet, which the manufacturers used to 

 make of a brownish yellow (they now make it of a bluish or 



