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THE POPULAR SCIENCE MONTHLY 



been explained by the investigations of the last few years. When cooled 

 slowly the steel undergoes a transformation changing to a form very 

 much softer than that which existed at a higher temperature. If chilled 

 suddenly the steel remains in the same form that was stable at high tem- 

 peratures, consequently the property of hardness is retained. Here 

 again the analogy to the other cases of delayed transformation is evi- 

 dent, for the quenched steel is exhibiting the same state of passive re- 

 sistance as the white tin that remains unchanged at a temperature below 

 18°. Now since the tin is not permanent under those conditions the 

 question occurs to us, does steel slowly return to the stable form and thus 

 in time grow softer ? That we do not know ; we can only say that if such 

 a change does take place hundreds of years are necessary to bring it 

 about. Japanese swords hardened in this way and made as far back as 

 the fifteenth century when carefully preserved are apparently as hard as 

 ever. If, however, this kind of steel is heated to the temperature of 

 boiling water it gradually softens, reverting to the stable form. And if 

 heated to 150° the softening takes place in a very few minutes. 



From these examples of retarded transformation an idea of the ex- 

 tent and the importance of this phenomenon in the physical sciences 

 may be obtained. New cases are constantly being discovered, in fact, 

 the reluctance of substances to assume a new state seems to be pretty 

 general. And as it so often happens in science that discoveries which 

 seem at first to be of theoretical importance only, ultimately are shown 

 to be intensely practical, so the study of this phenomenon has cleared 

 up the mystery of the tin pest and promises to be of great importance 

 in the study of metallurgy and many other branches of applied 

 chemistry. 



