READING COINS IN THE DARK. 117 



heit.* 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 surround- 

 ing 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 covered with black oxide are more luminous 

 than the adjacent parts, on account of the superior 

 radiation of light by the black oxide which covers 

 them. 



By the means now described, invisible writing 

 might be conveyed by impressing it upon a me- 

 tallic surface, and afterwards erasing it by grind- 

 ing and polishing that surface perfectly smooth. 

 When exposed to a proper degree of heat, the 

 secret would display itself written in oxidated 

 letters. Many amusing experiments might be 

 made upon the same principle. 



A series of curious and sometimes alarming 



* See Edinburgh Encyclopaedia, Art. STEEL, vol. xviii., 

 p. 387. 



