Purple Tint of Plate-glass affected by Light. [1823. 



Action of Gunpowder on Lead*. 



MR. MARSH gave me also some balls from cartridges about 

 fifteen years old, and which had probably been in a damp 

 magazine. They were covered with white warty excrescences 

 rising much above the surface of the bullet, and which, when 

 removed, were found to have stood in small pits formed be- 

 neath them. These excrescences consist of carbonate of lead, 

 and readily dissolve with effervescence in weak nitric acid, 

 leaving the bullet in the corroded state which their formation 

 has produced. It is evident there must have been a mutual 

 action amongst the elements of the gunpowder itself, at the 

 same time that it acted on the lead ; and it would have been 

 interesting, had the opportunity occurred, to have examined 

 what changes the powder had suffered. 



Purple Tint of Plate-glass affected by Light ^. 



IT is well known that certain pieces of plate-glass acquire, by 

 degrees, a purple tinge, and ultimately become of a comparatively 

 deep colour. The change is known to be gradual, but yet so 

 rapid as easily to be observed in the course of two or three years. 

 Much of the plate-glass which was put a few years back into 

 some of the houses in Bridge Street, Blackfriars, though at first 

 colourless, has now acquired a violet or purple colour. Wish- 

 ing to ascertain whether the sun's rays had any influence in 

 producing this change, the following experiment was made : 

 Three pieces of glass were selected, which were judged capable 

 of exhibiting this change ; one of them was of a slight violet 

 tint, the other two purple or pinkish, but the tint scarcely per- 

 ceptible, except by looking at the edges. They were each 

 broken into two pieces ; three of the pieces were then wrapped 

 up in paper and set aside in a dark place, and the correspond- 

 ing pieces were exposed to air and sunshine. This was done 

 in January last, and the middle of this month (September) they 

 were examined. The pieces that were put away from light 

 seemed to have undergone no change ; those that were ex- 

 posed to the sunbeams had increased in colour considerably ; 



* Quarterly Journal of Science, xvi, 163. t Ibid, 164. 



