260 



Mr. S. R. Roget. Effects of prolonged 



1 per cent. The curves have been sketched by joining the observed 

 points instead of drawing smooth curves through them, as this avoids 

 confusion of points belonging to different curves. 



It was found however that at higher temperatures, from about 

 135 C. upwards, a maximum value of the hysteresis was attained 

 in a comparatively short time, after which continued heating caused 

 a marked decrease of hysteresis instead of a further increase. The 

 initial rise at the higher temperatures is very rapid ; for example, 

 the hysteresis doubles in a few hours at a temperature of 160 C., 

 and reaches nearly three times its initial value in a few days. 

 Curve 5 of fig. 1 exhibits this case, the data for which are given in 

 Table I. After seven days of heating, the hysteresis of this sample 

 began to decrease, and in fifteen days it had fallen to 2^ times 

 its original value. A still more notable decrease occurs at higher 



JIG. 1. 



20O 



Fl 



