456 



Mr. J. Hopkinson. 



[Mar. 21, 



cylinder of hard steel, 6*3 cm. long and 5*1 cm. diameter, cut a 

 groove in it, and wrapped in the groove a copper wire insulated with 

 asbestos. The cylinder was wrapped in a large number of coverings 

 of asbestos paper to retard its cooling, the whole was then heated to 

 a bright redness in a gas furnace, was taken from the furnace and 

 allowed to cool in the open air, the resistance of the copper wire being 

 from time to time observed. The result is plotted in the accom- 

 panying curve, in which the ordinates are the logarithms of the 

 increments of resistance above the resistance at the temperature of 

 the room, and the abscissas are the times. If the specific heat of the 

 material were constant, and the rate of loss of heat were proportional 

 to the excess of temperature, the curve would be a straight line. It 

 will be observed that below a certain point this is very nearly the 

 case, but that there is a remarkable wave in the curve. The tempe- 

 rature was observed to be falling rapidly, then to be suddenly 

 retarded, next to increase, then again to fall. The temperature 

 reached in the first descent was 680 C. The temperature to which 

 the iron subsequently ascends is 712 C. The temperature at which 

 another sample of hard steel ceased to be magnetic, determined in 

 the same way by the resistance of a copper coil, was found to be 

 690 C. This shows that within the limits of errors of observation 

 the temperature of recalescence is that at which the material ceases 

 to be magnetic. This curve gives the material for determining the 



