THERMAL PHENOMENA 67 



pered " for too long a time or at too low or too high, 

 a temperature. All these processes are the direct 

 outcome of the purely scientific study of the 

 structure and constitution of metals and alloys. 



This aspect of our subject is so important that 

 it is worth while following it back to its beginnings. 

 These lie, as has already been indicated, in the 

 combination of the thermal study of metals, by 

 means of the Le Chatelier thermo-couple, with 

 the study of the micro-structure. In its simplest 

 form the thermal study of metals and alloys 

 consists merely in determining the melting or 

 freezing points. Even these, when taken for a 

 whole series of alloys between two metals and 

 plotted in the form of a " melting-point " curve, 

 form a diagram which is of considerable interest. 

 This melting-point curve, however, is but the 

 first step in an investigation of such a series of 

 alloys, because it is found that at temperatures 

 very much below the melting-point many metals 

 and alloys exhibit thermal phenomena of a most 

 interesting kind. The best-known of these occurs 

 in ordinary carbon steel. If a piece of such steel 

 is heated to bright redness and then allowed to 

 cool, it is seen to cool down steadily until apparently 

 just below visible redness and then the tempera- 

 ture suddenly rises again and a brief but bright 

 red glow passes over the steel. This is a striking 

 example of the kind of phenomenon which is by 

 no means uncommon in metals and alloys on 



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