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



Subsequent experiments with five of these kinds (all which 

 I could conveniently obtain) showed that an exposure of a year, 

 or even less, would change all but an ordinary kind of American 

 sheet, which was of a dark bluish-green colour. 



The experiments which speak for themselves are the most sa- 

 tisfactory ones — -that is to say, where one has not only the record 

 made at the end of each month, but a piece of glass taken in and 

 laid aside at the same time to show the actual colour produced, 

 and the truth of the records. It is very interesting to witness 

 any one of these series of specimens showing, as in one of white 

 plate, a gradual change (commencing in a day or a few days in 

 summer) from greenish or bluish white to a yellowish white, or 

 light yellow, a deep and deeper yellow, until it becomes a dark 

 yellow or a gold colour, — and in some Belgian sheet specimens 

 a gradual change (commencing in a few weeks in summer) from 

 brownish yellow to deeper yellow, yellowish pink, pink, dark 

 pink, purple, and deep purple. 



There are several kinds of glass in which no perceptible change 

 took place in three months, which were very sensibly affected by 

 an exposure of a year. 



Experiments. — I have given a general account of my first experi- 

 ments in 1863, and a portion of the Tables kept in my journal 

 at that time. I might have given names and results in full, 

 and shown the actual effects and shades of colour produced by 

 exposure for a few months on some thirty kinds of glass. But 

 in my case, as in many novel and original investigations, the 

 results of first experiments, and the theories based upon them, 

 were modified by subsequent ones. I supposed that many 

 kinds of glass not changed in three months would not change 

 at all, that all which changed would take a yellowish colour, 

 unless by exposure of many years, that no colour but some 

 shade of yellow or pink would ever be produced in any kind by 

 exposure to sunlight. 



The experiments of 1864 and the two following years proved 

 to me that nearly every kind of window-glass I had exposed could 

 be changed in one year, that a rose or pink colour (or some tint 

 approaching them) could be produced in various kinds in a few 

 months, and that some kinds of greenish-white glass would, 

 after exposure, assume a bluish tint or bluish white. 



It may seem singular for one who has been a glass dealer and 

 manufacturer like ourselves, thus to advertise what may be called 

 a defect in his own wares. It might seem unkind to other manu- 

 facturers to expose the defects of their productions literally before 

 the light of day. But my scruples were all removed when I 

 noticed, in a late communication of Pelouze (see Comptes 

 Rendus } January 14, 1867), the following statement : — " I do 



