Mr. T. Gaffield on the Action of Sunlight on Glass. 529 



upon glass thus made, the nice equilibrium between the oxygen 

 of the iron and the manganese is disturbed, and sometimes the 

 yellow and sometimes the pink or purple colour is produced. I 

 have produced all shades of the purples, running from pale la- 

 vender into the lilac, mulberry, flesh, amethyst, rose, violet, pink, 

 and deep durple. I have produced, or seen, specimens showing 

 all shades of the yellow, from the brownish yellow up to the 

 brightest gold-colour; and I have several series of specimens in 

 which the green has gradually changed into the yellow, and the 

 yellow gradually run into the pink and purple. 



Pelouze, in an article in the Comptes Rmdus of January 14, 

 1867, sets forth the following theory : — 



" There is in glass coloured yellow in sunlight some protoxide 

 of iron and sulphate of soda. Light provokes between these 

 matters a reaction, from which results peroxide of iron and sul- 

 phide of sodium. The heat brings about an inverse reaction, 

 and reproduces sulphate of soda and protoxide of iron. From 

 thence comes the return of the glass to its primitive colour. 

 Analysis conies to the aid of this theory, in demonstrating in 

 glass rendered yellow by sunlight the presence of an infinitely 

 feeble but still very sensible proportion of a sulphide, whilst the 

 reactions do not show the slightest trace in the same glass before 

 its exposure. 



" It may be asked why glasses coloured by the reduction of 

 the sulphate, or the direct introduction of a sulphide into their 

 mass, resist an equal or superior heat to that which produces the 

 decoloration of glass become yellow in sunlight. Here is the 

 answer. In glass made yellow at a high temperature by the re- 

 duction of sulphate, the iron is found in a state of protoxide, 

 which cannot react in any degree upon the sulphides. Therefore 

 the glass remains coloured. In glass made yellow by sunlight 

 the iron is peroxide, and in consequence in a condition to change 

 the sulphide into sulphate when we expose the glass to the 

 action of heat." 



In reference to glass made violet-colour by sunlight, he says, 

 " The coloration seems to be due to the fact that the peroxide 

 of iron gives up a part of its oxygen to the protoxide of manga- 

 nese, conformably to one of the two following equations : — 



Fe 2 O 3 + MnO = 2 (FeO) -f MnO 2 , 

 or 



Fe 2 O 3 + 2 (MnO) = 2 (FeO) + Mn 2 O 3 . 

 " The reheating of glass (that is to say, the action of a tempe- 

 rature of red heat) produces an inverse reaction, which explains 

 the decoloration, in which we have 



2 (FeO) + Mn 2 ? ' - Fe 2 O 3 + 2 (MnO) ." 



