Mr. T. Gaffield on the Action of Sunlight on Glass. 525 



except a very slight one in a few specimens which were burnt or 

 overheated. 



In PoggendorfFs Annalen for May 1839, is recorded the fol- 

 lowing interesting fact by A. Splittgerber : — 



" I would mention a curious fact in which the sunbeams have, 

 if I may say so, done something in the art of penmanship, not 

 only on the surface, but by inscribing characters through the 

 body of the glass ; and though the matter is based upon causes 

 well known by experience, yet there has probably never before 

 been so striking an instance of their effect known. I am in 

 possession of a plate of glass which was used as a window-pane 

 for more than twenty years, and on which was an inscription in 

 gold letters. This inscription was taken off by grinding the 

 plate on both sides, and polishing it so as to have a new surface. 

 When the glass had been polished, the inscription could again 

 be clearly seen. The parts which had been under the letters 

 remained white, while the remainder of the plate had assumed 

 a violet tint in consequence of the manganese it contained — a 

 colouring which permeates the whole mass, as the grinding of 

 the surface proved. The uncovered part of the plate, especially 

 when laid upon a white background, shows the clearly readable 

 characters." 



The same, or a similar instance, is related by Dr. Hermann 

 Vogel in the Photographische Mittheilungen, September 1866. 



Desiring to produce a similar result, we made an inscription 

 on a piece of Belgian sheet glass in part with gold- and silver- 

 leaf, and in part with black and white paint. The gold- and 

 silver-leaf were soon washed off, but the black- and white-painted 

 letters remained, and being removed after an exposure of nearly 

 two years, the words stood out in clear contrast and full propor- 

 tions — the inscription being in the original colour of the glass, 

 and the surrounding portions having been changed by the action 

 of the sunlight to a purple colour. 



A very interesting experiment can be made to show the gradu- 

 ally increasing effect of the sunlight on glass, by taking a piece 

 of easily changing glass, say 4 x 20 inches, painting black a 

 strip 4x2 inches at each end to preserve the original colour, 

 and then exposing the strip to sunlight. At the end of one, two, 

 four, six, eight, and ten months, one, two, and three years re- 

 spectively, cover with black paint a strip 4x2 inches, and at 

 the end of three years remove all the paint, and you will have 

 in a single piece of glass the original colour and all the grada- 

 tions of change effected by exposure from one to thirty-six 

 months. I have made a similar one with Belgian sheet glass 

 exposed nearly two years. It is one of those interesting expe- 

 riments which speak for themselves and defy suspicion or con- 

 tradiction. 



