116 LETTERS ON NATURAL MAGIC. 



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 become 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 a uniform and homogeneous disc of 

 silver that has never been hammered or com- 

 pressed, its surface will oxidate equally, provided 

 all its parts are equally heated. In the process 

 of converting this disc into a coin, the sunk parts 

 have obviously been most compressed by the 

 prominent parts of the die, and the elevated parts 

 least compressed' the metal being in the latter left 

 as it were in its natural state. The raised letters 

 and figures on a coin have therefore less density 

 than the other parts, and these parts oxidate 

 sooner or at a lower temperature. When the 

 letters of the legend are worn off by friction, the 

 parts immediately below them have also less 

 density than the surrounding metal, and the site 

 as it were of the letters therefore receives from 

 heat a degree of oxidation, and a colour different 

 from that of the surrounding surface. Hence we 

 obtain an explanation of the revival of the invisi- 

 ble letters by oxidation. 



The same influence of difference of density 

 may be observed in the beautiful oxidations 

 which are produced on the surface of highly- 

 polished steel, heated in contact with air, at 

 temperatures between 430 and 630 of Fahren- 



