THE SIZE OF ATOMS. 175 



You are all familiar with the brilliant and 

 beautifully distributed fringes of heat-colours on 

 polished steel grates and fire-irons escaping that 

 unhappy rule of domestic aesthetics which too 

 often keeps those articles glittering and cold and 

 useless, instead of letting them show the exquisite 

 play of warm colouring naturally and inevitably 

 brought out when they are used in the work 

 which is their reason for existence. The thick- 

 ness of the film of oxide which gives the first 

 perceptible colour, a very pale orange or buft 

 tint, due to the enfeeblement or extinction of 

 violet light and enfeeblement of blue and less 

 enfeeblement of the other colours in order, by 

 interference of the reflections from the two sur- 

 faces of the film, is about i/ioo,oooth of a centi- 

 metre, being something less than a quarter wave- 

 length of violet light. 



The exceedingly searching and detective efficacy 

 of electricity comes to our aid here, and by the 

 force, as it were, spread through such a film, 

 proves to us the existence of the film when it is 

 considerably thinner than that i/ioo,oooth of a 



