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 temperatures, consequently the property of hardness is retained. Here again the analogy to the other cases of delayed transformation is evident, for the quenched steel is exhibiting the same state of passive resistance 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 extent 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.
