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



Bontemps, in an interesting article in the Comptes Rendus of 

 February 4, 1867, attributes the changes in colour to the pre- 

 sence of oxide of manganese. He also remarks that he thinks 

 that the violet colour occurs only in glass in which the silicates 

 have a base of potash, and the yellow in cases where soda is 

 used. I doubt the correctness of this opinion, as I am quite 

 sure that I have several specimens coloured violet which contain 

 no potash at all in their composition. 



I have also specimens which are coloured both yellow and 

 violet in the same piece — the yellow portion in one case having 

 been produced by a certain length of exposure, and then having 

 been covered with black paint while the violet portion was pro- 

 duced by longer exposure. 



As before stated, I have noticed changes in what are called 

 colourless glasses, from light colours approaching white to yel- 

 low and pink or purple. I have noticed also a change in a few 

 specimens from a light-green to a bluish shade. The former 

 may be accounted for by the presence of manganese, a very 

 minute proportion of which oxide will have a sensible decolori- 

 zing effect in a crucible of melted glass-metal. Pelouze J s theory 

 of the peroxidation of the iron may have some weight in the deter- 

 mination of the cause of the yellow or pink colour by the action 

 of sunlight. But I know not how to account for the change of 

 one specimen each of plate, crown, and sheet glass in my pos- 

 session, from a greenish white to a bluish tinge not mingled with 

 either yellow or purple. 



I have been pleased to find the interest taken in these experi- 

 ments by photographers, who have long noticed that they could 

 take better pictures under a newly-glazed skylight than under 

 one which had long been exposed to sunlight. The cause of this 

 change is that the slightest yellow colour interferes with the 

 transmission of the actinic rays, and a very deep shade will cut 

 them off in a very great degree. My experiments with glasses 

 under other glasses proved which was best for photographers' 

 use, information which all can gain by exposure of the speci- 

 mens of various manufacturers which may be offered them. 

 The most pure glasses of light-green or bluish-white colour are 

 the best for photographers ; and when I say pure glasses, I mean 

 those most free from oxide of iron or manganese, but especially 

 of the latter, which I think is the cause of nearly all the changes 

 which I have observed. Mr. J. W. Osborne, of New York, 

 the gentleman who has done so much to bring the art of photo- 

 lithography to perfection and into practical use, writes as fol- 

 lows : — 



" I believe your researches will prove of much practical im- 

 portance, and I wish the glass-manufacturers could be got to 



