EVOLUTION AND MUTABILITY OF MATTER. 277 



another point. If another rest is given at this point 

 the metal will also become hardened. 



If we repeat the experiment a sufficient number of 

 times, we shall find a total transformation of the rod, 

 which becomes hardened throughout its entire extent. 

 It will break rather than elongate if the stretching is 

 sufficiently severe. 



Nickel Steels — tJieir ^'Heroic'"' Resistance. — Nickel 

 steels present this phenomena in an exaggerated 

 degree. The alternation of operations which we 

 have just described, bringing the various parts of an 

 ordinary steel rod into a tempered state, is not 

 necessary with nickel steel. The effect is produced 

 in the course of a single trial. As soon as there is 

 any tendency to contraction the alloy hardens at that 

 precise place ; the contraction is hardly noticeable ; 

 the movement is stopped at this point to attack 

 another weak point, stops there again and attacks a 

 third, and so on ; and, finally, the paradoxical fact 

 appears that a rod of metal which was in a soft state 

 and could be considerably elongated has now become 

 throughout its whole extent as hard, brittle, and 

 inextensible as tempered steel. It is in connection 

 with this point that M. C. E. Guillaume spoke of 

 ••heroic resistance to rupture." It would seem, in 

 fact, as if the ferro-nickel bar had reinforced each 

 weak point as it was threatened. It is only at the 

 end of these efforts that the inevitable catastrophe 

 occurs. 



Effect of Temperature. — When the temperature 

 changes, it is seen that these ferro-nickel bars elongate 

 or retract, modifying at the same time their chemical 

 constitution. But these effects, like those which occur 

 in the glass bulb of a thermometer, do not occur 



