READING- COINS IN THE DARK. 189 



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 

 invisible 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 

 Fahrenheit.* When the steel has hard portions called 

 pins by the workmen, the uniform tint of the film of oxide 

 stops near these hard portions, which always exhibit 

 colours different from those of the rest of the mass. 

 These parts, on account of their increased density, absorb 

 the oxygen of atmospheric air less copiously than the 

 surrounding portions. Hence we see the cause why steel 

 expanded by heat absorbs oxygen, which, when united 

 with the metal, forms the coloured superficial film. As 

 the heat increases, a greater quantity of oxygen is absorbed, 

 and the film increases in thickness. 



These observations enable us to explain the legibility 

 of inscriptions in the dark, whether the coin is in a perfect 

 state, or the letters of it worn off. All black or rough 

 surfaces radiate light more copiously than polished or 

 smooth surfaces, and hence the inscription is luminous when 

 it is rough, and obscure when it is polished, and the letters 



* See Edinburgh Encyclopedia, Art. STEEL, vol. xviii. p. 387. 



