READING COINS IN THE DARK. 115 



heat. The parts, however, where the letters of 

 the inscription had existed, oxidate at a different 

 rate from the surrounding parts, so that these 

 letters exhibit their shape, and become legible in 

 consequence of the film of oxide which covers 

 them having a different thickness, and therefore 

 Fig. 23. Fig. 24. 



reflecting a different tint from that of the adjacent 

 parts. The tints thus developed sometimes pass 

 through many orders of brilliant colours, par- 

 ticularly pink and 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, how- 

 ever, in the course of time. When the coin is 

 put upon the hot iron, and consequently when 

 the oxidation is the greatest, a considerable smoke 

 arises from the coin, and this diminishes like the 

 I 2 



