110 Mr. Herbert Tomlinson on 



original positions, just as water begins to freeze when the 

 temperature of 0° C. has been reached ; but internal friction 

 prevents this, and it is not till the temperature has fallen, it 

 may be very considerably, below the critical temperature that 

 the readjustment takes place. When, however, the change 

 under these conditions does take place it will be what Clerk 

 Maxwell has designated as an explosive change, and the energy 

 set at liberty by the transformation will accelerate the sub- 

 sequent rate of transformation. Here, again, we have an 

 analogy in the case of freezing water; for in a Wollaston's 

 cryophorus the temperature of the water may frequently be 

 reduced several degrees below 0° C. before solidification sets 

 in ; but when it does, the whole mass, or a considerable por- 

 tion of the whole mass, is frozen instantly. With both iron 

 and water, therefore, there will be a greater amount of heat 

 suddenly generated when the transformation takes place below 

 the critical temperatures than when it occurs at the critical 

 temperatures. Again, another analogy may be pointed out 

 between ice at 0° C. and iron at 1000° C. ; in both there is a 

 marked change in the cohesion of the molecules. When an 

 iron wire is under any but an exceedingly small stress of 

 bending, torsion, or traction, it begins to yield permanently 

 in a most astonishing manner when, on heating, the tempera- 

 ture reaches 1000° C. ; the metal suddenly becomes, as it 

 were, in a partially fluid condition. 



According to the author's view, then, recalescence in cooling 

 iron is owing to retardation, produced by internal friction, of 

 a physical change somewhat resembling the change which 

 occurs when water becomes ice : were it not for internal fric- 

 tion the change would take place comparatively in a gradual 

 manner, and a certain amount of thermal energy would be 

 equally gradually given out. In consequence, however, of 

 internal friction the change is retarded until a temperature is 

 reached below the temperature at which it would otherwise 

 occur ; when, partly owing to diminution of internal friction 

 and partly to increased internal molecular stress, the mole- 

 cules give way at some one point, and this is followed by a 

 giving way throughout the whole mass ; the change thus 

 partakes of the nature of an explosion, and. as a consequence, 

 there is a rapid rise of temperature. The following experi- 

 ments were made with the object of collecting evidence for or 

 against the above-mentioned theory. 



Experiments on Recalescence. 

 It has been observed that there are hco critical tempera- 

 tures — one at about 550° C, and the other at about 1000° C. ; 



