188 



LETTERS ON NATURAL MAGIC. 



green, and settle in a bronze, and sometimes a black tint, 

 resting upon the inscription alone. In some cases the 

 tint left on the trace of the letters is so very faint that it 

 can just be seen, and may be entirely removed by a slight 

 rub of the finger. 



When the experiment is often repeated with the same 

 coin, and the oxidations successively removed after each 

 experiment, the film of oxide continues to diminish, and at 

 last ceases to make its appearance. It recovers the 

 property, however, in the course of time. When the coin 

 is put upon the hot iron, and consequently when the oxi- 

 dation is the greatest, a considerable smoke arises from 



Fir. 24, 



the coin, and this diminishes, like the film of oxide, by 

 frequent repetition. A coin which had ceased to emit 

 this smoke, smoked slightly after having been exposed 

 twelve hours to the air. I have found from numerous 

 trials that it is always the raised parts of the coin, and in 

 modern coins the elevated ledge round the inscription, 

 that becomes first oxidated. In an English shilling of 

 1816 this ledge exhibited a brilliant yellow tint before it 

 appeared on any other part of the coin. 



If we use an uniform and homogeneous disc of silver 

 that has never been hammered or compressed, its surface 

 will oxidate equally, provided all its parts are equally 

 heated. In the process of converting this disc into a coin. 



