Jan. 23, 1890] 



NATURE 



275 



great difficulty in the case of soft iron, and 

 is not observed at all in the case of man- 

 ganese steel. A fairly approximate numeri- 

 cal measurement may be made in this 

 way : Take a block of iron or steel on 

 which a groove is cut, and in this groove 

 wind a coil of copper wire insulated with 

 asbestos ; cover the coil with many layers 

 of asbestos ; and finally cover the whole 

 lump of iron or steel with asbestos again. 

 We have now a body which will heat and 

 cool comparatively slowly, and which will 

 lose its heat at a rate very approximately 

 proportional to the difference of tempera- 

 ture between it and the surrounding air. 

 Heat the block to a bright redness, and 

 take it out of the fire and observe the 

 resistance of the copper coil as the 

 temperature falls, due to the cooling of 

 the block. Plot a curve in which the 

 abscissae are the times, and the ordinates 

 the logarithms, of the increase of resist- 

 ance of the copper coil above its resistance 

 at the temperature of the room. If the 

 specific heat of the iron were constant, 

 this curve would be a straight line ; if 

 at any particular temperature latent heat 

 were liberated, the curve would be hori- 

 zontal so long as the heat was being 

 liberated. If now a block be made of 

 manganese steel, it is found that the 

 curve is very nearly a straight line, show- 

 ing that there is no liberation of latent heat 

 at any temperature. If it is made of 

 nickel steel with 25 per cent, of nickel, in 

 its non-magnetic state, the result is the 

 same — no sign of liberation of heat. If 

 now the block be made of hard steel, 

 the temperature diminishes at first ; then 

 the curve (Fig. 13) which represents the 

 temperature bends round : the tempera- 

 ture actually rises many degrees whilst 

 the body is losing heat. The liberation 

 of heat being completed, the curve finally 

 descends as a straight line. From in- 

 spection of this curve it is apparent why 

 hard steel exhibits a sudden accession of 

 brightness as it yields up its heat. In 

 the case of soft iron the temperature does 

 not actually rise as the body loses heat, 

 but the curve remains horizontal, or nearly 

 horizontal, for a considerable time. This, 

 again, shows why, although a consider- 

 able amount of heat is liberated at a 

 temperature corresponding to the hori- 

 zontal part of the curve, no marked re- 

 calescence can be obtained. From curves 

 such as these it is easy to calculate the 

 amount of heat which becomes latent. 

 As the iron passes the critical point it 

 is found to be about 200 times as much 

 heat as is required to raise the tempera- 

 ture of the iron i degree Centigrade. 

 From this we get a very good idea of 

 the importance of the phenomenon. 

 When ice is melted and becomes water, 

 the heat absorbed is 80 times the heat 

 required to raise the temperature of 

 the water 1 degree Centigrade, and 

 160 times the heat required to raise 

 the temperature of the ice by the 

 same amount. The temperature of re- 

 calescence has been abundantly identi- 

 fied with the critical temperature of 



Fl3. 13. 



