68 MODERN SCIENCE OF METALS 



cooling down to some definite temperature an 

 evolution of heat occurs which temporarily arrests 

 the rapid fall of temperature, or in a few special 

 cases may be sufficiently violent actually to 

 raise the temperature again. While in steel this 

 effect is strong enough to be visible to the eye, in 

 many metals and alloys it is necessary to employ 

 the thermo-couple with the most delicate electrical 

 measuring instruments in order to detect and 

 locate these "transformation" points. When all 

 such points have been observed in an alloy series, 

 it becomes possible to plot the results in the form 

 of a diagram known as the "equilibrium" or 

 "constitutional" diagram, which represents in a 

 graphic form all the changes and different condi- 

 tions to which the various alloys of that system are 

 subject at different temperatures. Such a diagram 

 cannot, however, be satisfactorily drawn solely 

 from observations made with the thermo-couple; 

 it is necessary to call in the aid of the microscope 

 to explain the real meaning of the various lines 

 and areas of these diagrams. This is possible 

 because it is found that with practically every 

 such "transformation" or "critical" point there 

 is associated a well-marked change of micro- 

 structure. It is true that as a rule we can only 

 study the structure of a piece of metal after it has 

 been allowed to cool down, but fortunately it is 

 found that if a specimen is "quenched" or chilled 

 very rapidly it can be more or less completely 



