AND INDUSTRY 19 



study of the thermoelectric properties of 

 metals due to Seebeck, Peltier, and William 

 Thomson. Petrographers had been in the 

 habit of examining the structure of rocks by 

 cutting sections thin enough to be trans- 

 parent and examining them under the micro- 

 scope. Sorby in 1861 found it was not 

 possible to examine metals thus and developed 

 the art of polishing the surface and etching 

 it with suitable chemicals, thus bringing out 

 the internal structure. Its application to 

 engineering problems passed unnoticed until 

 the method was independently revived by 

 Osmond in France and Martens in Germany. 

 Seebeck discovered that when in a circuit of 

 two metals a difference of temperature exists 

 between the junctions, an electric current is 

 produced in the circuit. The strength of this 

 current is a measure of the difference in tem- 

 perature, and this discovery was applied many 



