276 LIFE AND DEATH. 



in the same way, too, in the manufacture of steel, 

 the particles of coal at first applied to the surface 

 pass through the iron. 



This faculty of molecular displacement enables the 

 metal in some cases to modify its state at one point 

 or another. The use made of this faculty under 

 certain circumstances is very curious, greatly re- 

 sembling the adaptation of an animal to its environ- 

 ment, or the methods of defence against agents that 

 might destroy it. 



Ejfect of Stretching. Hartinann's Experiment. — 

 When a cylindrical rod of metal, held firmly at either 

 end — a test-piece, as it is called in metallurgy — is 

 pulled "sufficiently hard, it often elongates consider- 

 ably, part of the elongation disappearing as soon as 

 the strain ceases, and the other part remaining. The 

 total elongation is thus the sum of an " elastic 

 elongation," which is temporary, and a " permanent 

 elongation." If we continue the stretching, there 

 appears at some point of the rod a local extension 

 with contraction of sectional area. It is here that the 

 rod will break. 



But in place of continuing the stretching, Mr. 

 Hartmann suspends it. He stops, as if to give the 

 "metal-being" time to rally. During this delay it 

 would seem that the molecules hasten to the menaced 

 point to reinforce and harden the weak part. In 

 fact the metal, which was soft at other points, at 

 this spot looks like tempered metal. It is no longer 

 extensible. 



When the experimenter begins the stretching again 

 after this rest, and after the narrowed bar has been 

 rolled and become cylindrical again, the local ex- 

 tension and secUonal contraction is forced to occur at 



