414 THE LIFE <)t MATTER. 



more subject to tliese inoditication.s than those having a more simple 

 composition based on a single alkali. 



Effeds of annealing. — A piece of brass wire that has been drawn and 

 then heated is the theatre of very remarkable internal changes that 

 liaA^e been only recently recognized. The violence done to the metallic 

 thread in forcing it through the hole in the die crushed the crystalline 

 particles; the interior state of the wire is that of broken crystals sur- 

 rounded by a gj-anular mass. Heating changes all that. The crystals 

 separate, repair themselves, and reform; they are then hard, geomet- 

 rical bodies, plunged in an amorphous mass, relatively soft and plastic; 

 their number keeps on increasing; equilil)rium is not established until 

 the entire mass is crystallized. One may imagine how many displace- 

 ments, enormous when compared with their dimensions, the molecules 

 have to undergo when transporting themselves throughout the resisting 

 mass and arranging themselves in definite places in the crystalline 

 edifices. 



In the sanie way, too, in the manufacture of steel, the particles of 

 coal at first applied to the surface tra\'erse the iron. 



This faculty of molecular displacement permits in some cases the 

 metal to modify its state at one point or another. The use made of 

 this faculty in certain circumstances is very curious, greatly resem- 

 bling the adaptation of an animal to its environment, or the methods 

 of defense which it employs to resist agents that might destroy it. 



Effect of fraefhyn and str lei ion — Exmrhnent of Hartniann . — When a 

 cylindrical rod of metal, held firmly at either end — a test rod, as it 

 is called in metallurgy — is subjected to a powerful traction it often 

 elongates considerably, part of the elongation disappearing as soon as 

 the strain stojjs, another part remaining. The total elongation is thus 

 the sum of an "elastic elongation," which is temporary, and a "per- 

 manent elongation." If we continue the traction there appears at 

 some point of the rod a narrowing, a striction. It is there that the 

 rod will l)reak. 



But in place of continuing the traction. 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 has here taken on the aspect of tempered metal. 

 It is no longer extensible. 



When the experimenter reconunences traction after this rest, the 

 narrowed bar having been submitted to the action of a roller and 

 returned to a cylindrical form, a second narrowing forms at another 

 point. If another rest is given this striction also will become resistant. 



If we renew this experiment a sutficient number of times there will 

 result a total transformation of the rod, which will become hardened 

 throughout its entire extent. It will break rather than elongate if 

 the traction is sutficientlv energetic. 



