READING COINS IN THE DARK. 187 



depressed parts, and we shall still be able to read it, from 

 its being as it were written in black letters on a white 

 ground. The first time I made this experiment, without 

 being aware of what would be the result, I used a French 

 shilling of Louis XV., and I was not a little surprised to 

 observe upon its surface in black letters the inscription 

 BENEDICTUM SIT NOYEN DEI. 



The most surprising form of this experiment is when 

 we use a coin from which the inscription has been either 

 wholly obliterated, or obliterated in such a degree as to 

 be illegible. When such a coin is laid upon the red-hot 

 iron, the letters and figures become oxidated, and the film 

 of oxide radiating more powerfully than the rest of the 

 coin will be more luminous than the rest of the coin, and 

 the illegible inscription may be now distinctly read, to the 

 great surprise of the observer, who had examined the 

 blank surface of the coin previous to its being placed upon 

 the hot iron. The different appearances of the same coin, 

 according as the raised parts are polished or roughened, 

 are shown in Figs. 23 and 24. 



In order to explain the cause of these remarkable 

 we must notice a method which has been long 



lown, though never explained, of deciphering the in- 

 scriptions on worn-out coins. This is done by merely 

 placing the coin upon a hot iron. An oxidation takes 

 place over the whole surface of the coin, the film of oxide 

 changing its tint with the intensity or continuance of the 

 heat. The parts, however, where the letters of the inscrip- 

 tion had existed oxidate at a different rate from the sur- 

 rounding 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 there- 

 fore reflecting a different tint from that of the adjacent 

 parts. The tints thus developed sometimes pass through 

 lany orders of brilliant colours, particularly pink and 



m$ 



