464 
DR. J. HOPKINSOJf ON MAGNETIC AND OTHER PHYSICAL 
to some change in the materia], and not due to any change in the conductivity or 
emissive power. This has indeed been satisfactorily proved by Mr, Newall.* My 
method of experiment was exceedingly simple. I took a cylinder of hard steel 
6'3 cms. long and 5T cms. in 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 Curve 
XXXVIII., in which the ordinates are the logarithms of the increments of resistance 
above the lesistance at the temperature of the room, and the abscissae 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 temj)erature, 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 temperature 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 quantity of heat liberated. 
The dotted lines in the curve show the continuation of the first and second parts of the 
cuiwe; the horizontal distance between these approximately represents the time during 
which the material was giving out heat without fall of temperature. After the bend 
in the curve, the temperature is falling at the rate of 0’21° C. per second. The 
* ‘ Phil. Mag.,’ June, 1888. 
