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



A similar experiment to the above I have commenced with 

 two pieces of white plate and the simple letters T. G. without 

 the date. The result of the experiment in a year or more will 

 be to show in one case yellowish letters on a light-coloured 

 ground, and in the other light-coloured letters on a yellowish 

 ground. 



I have thus given as briefly as possible, and yet as fully as 

 desirable, an account of my past and present experiments. New 

 ones are suggested from year to year. I trust that this inter- 

 esting field for observation and experiment may be worked in 

 other countries. There is ample room for research in the ap- 

 plication of chemical knowledge, of qualitative, quantitative, 

 aud spectral analysis, and of photogenic tests, to discover the 

 exact action and causes of the interesting effects of the sun's 

 rays which have here been noticed. 



Theories. — The interesting phenomena of which I have given 

 an account have given rise to many theories to account for their 

 cause. Some attribute them to the presence of oxide of iron, 

 and some to oxide of manganese. Exactly how the change 

 takes place is a question on which writers differ, although it is 

 my opinion that the precise explanation can only be given after 

 a multiplication of experiments and a thorough examination of 

 exposed and unexposed specimens of glass by quantitative and 

 qualitative analysis, and perhaps by spectral analysis and obser- 

 vation of effects, or photogenic tests*. 



We will briefly state the part which the oxides of iron and 

 manganese play in glass-making. In almost all kinds of win- 

 dow-glass, and in some poorer qualities of flint glass and glass- 

 ware, materials are used which are not perfectly and chemically 

 pure. The sand, the carbonate or sulphate of soda, and the 

 lime, one or all, contain slight impurities of iron, the protoxide 

 of which gives glass a green colour. To correct this after the 

 batch is partially melted, a little oxide of manganese, called 

 glass-makers' soap, is put into the crucible or glass-pot ; some 

 of the oxygen of the manganese flies off to the iron and con- 

 verts the protoxide into peroxide of iron. The peroxide gives a 

 yellowish colour to the glass ; and this, being complementary to 

 the natural pink of the manganese, is neutralized, and the glass 

 is thereby made of a light colour. When the sunlight acts 



* Since writing the above, by the kindness of Mr. John A. Whipple 

 (the distinguished photographer of Boston) I have been enabled to show, by 

 the comparative darkening of sensitive paper under several exposed and 

 unexposed specimens, the eifect of exposure to sunlight for one year. The 

 loss of actinic power, or power to transmit the actinic rays, was in propor- 

 tion to the change of colour. This was in some varieties of glass quite 

 perceptible, but in all will be more so after an exposure of several years. 



