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



" His inquiries, since he instituted these experiments, have 

 brought out some fine specimens of Belgian sheet glass from a 

 house built three years ago, which had changed in some instances 

 to a golden and in others to the well-known purple hue. 



i( It is his intention to pursue the experiments further, with a 

 view to ascertain the effects of sunlight during each month and 

 season of the year — and also whether exposure to heat, air, or 

 moisture alone, out of the direct action of the sun's rays, will 

 produce any corresponding change. 



" Mr. Gaffield does not propound any theory to explain these 

 changes of colour, which under our sunny skies probably take 

 place much more rapidly than in the different and less clear at- 

 mosphere of England. 



" Some writers point to the presence of oxide of manganese in 

 the original composition of window-glass, and some to the oxide 

 of iron, as a chief cause. 



" Some writers have peculiar theories about the different 

 classes of the sun's rays. Some may think the change referred 

 to a molecular or chemical one ; and others, wiser than the rest, 

 refrain from any explanation, waiting for a larger multiplication 

 of experiments and a greater accumulation of facts before edu- 

 cing any satisfactory law of nature which governs these curious 

 and interesting phenomena. 



" Mr. Gaffield makes no pretensions to any discoveries, unless 

 it be to the very rapid change in glass observed in our climate 

 in July, but only gives the result of his experiments in the hope 

 that the great interest now manifested in the subjects of light 

 and heat may lead others to examine the matter, to repeat the 

 same experiments in other countries, and to give the world the 

 result of their researches, and enable the learned and scientific 

 men of the age to explain this remarkable power and action of 

 the sun's rays. 



" It should be remembered that he submitted his specimens 

 to the most severe tests, by placing them where they received 

 reflected as well as transmitted light and heat. The change in 

 glass when glazed in the windows of our dwellings and stores 

 is so much more gradual that it very rarely attracts the atten- 

 tion of observers, except in the marked variation from white to 

 purple." 



In accordance with the intention above expressed, I have 

 continued my experiments on this interesting subject, and 

 under different heads will now give some account of their 

 method and results. 



My first experiment was with pieces of glass 4 by 6 inches 

 placed in a sash 6 by 60 inches long, in the grooves of which 



