350 BRUSH— DEVELOPMENT OF MAGNETIC SUSCEPTIBILITY 



alternating heating current. The higher temperatures usually oc- 

 curred in the latter part of the night, and were always accompanied 

 by more than average rise of susceptibility. But the large depres- 

 sion in the central part of the curve is thought to be due to some ob- 

 scure cause, and not to temperature variation. 



The entire absence of growth of susceptibility during the last 50 

 or more hours prompted the belief that the steel had reached a stable 

 condition at the temperature of treatment, and led to the discontinu- 

 ance of this experiment. Permanent magnetism, which had been 

 considerable while susceptibility was rising, fell off very much dur- 

 ing the last two or three days. 



Fifth quenching: At the end of 170 hours the bars were 

 quenched, after which they exhibited moderate, but typical and un- 

 equivocal generation of heat. 



Hardness was: Bar No. i, 48.1 ; Bar No. 7, 47.2. 



This great increase of hardness (from 2.6.^ and 25.8 in the an- 

 nealed condition) brought about by the long heating, doubtless ac- 

 counts for the spontaneous generation of heat observed. 



During the long heating the bars acquired a rather thick coating 

 of black oxide which peeled off almost completely in quenching, 

 leaving clean metal surface. The oxide was strongly magnetic ; but 

 its weight was so small, compared with the total weight of the bars, 

 that it could not have affected, materially, the foregoing magnetic 

 observations. 



The bars were again placed in the furnace and heated to a higher 

 temperature than before, fluctuating between 590° and 598°, for the 

 first 90 hours (from 170 to 260 hours, reckoned from beginning of 

 treatment). 



The results of this procedure are plotted in the first curve of 

 Fig. 2. It is seen that magnetic susceptibility started at a very con- 

 siderably higher value than it had at the end of the previous treat- 

 ment. The reason of this increase during the intervening few days, 

 without heating, is not clear. It may have occurred at the moment 

 of quenching; or, more likely, during the period of spontaneous heat 

 generation which followed the quenching. 



The curve shows a very regular, but steadily diminishing, growth 



