190 PROCEEDINGS OF THE AMERICAN ACADEMY. 



The temperature coefficient of the resistivity of the first rod spoken 

 of above was 0.00102 when the rod was soft; the third rod had a 

 temperature coefficient of 0.00094. 



Cast iron which has been several times hardened and annealed is 

 finally in its annealed state not so permeable as once-annealed soft 

 cast iron is. Table II gives the results of tests upon a rod of resis- 

 tivity 98.8 which has been four times heated white hot and chilled and 

 then annealed. 



If the process of heating and chilling a number of cast-iron rods be 

 carried out many times in succession without proper annealing after 

 each chilling, there does not seem to be a progressive increase in the 

 resistivity ; the results are anomalous. 



Several kinds of chilling baths were used for hardening the cast iron, 

 among them ice cold water, cold brine, sulphuric acid and water, an 

 acid bath (A') the constitution of which is a trade secret, but which, I 

 understand, has been much used in commercial work ; mineral oil, and 

 pa,raffine. 



It has long been known that in the hardening of tool steel from 

 a dull red heat, it is much more important that the fall of the tem- 

 perature of the piece down to say 300° C. shall be quickly brought 

 about than that the rest of the journey to room temperatures 

 shall be rapid. It is not difficult to cool quickly a slender rod, 

 but a large piece of hot metal suddenly immersed in a water bath 

 is immediately surrounded by a layer of steam and, unless the water be 

 very vigorously stirred as in die hardening, the metal may remain red 

 hot for a comparatively long time. Many attempts have been made by 

 varying the chemical nature of the bath to lessen the effect of the steam 

 cloak, and some persons have used a bath of easily fusible metal for the 

 first part of the chilling process (as is now the practice for some of the 

 new high power steels), and have completed the cooling in a water 

 bath, the temperature of which within wide limits seems to be 

 unimportant. 



In the light of the behavior of steel, it seemed unlikely that in the 

 hardening of cast iron from a temperature much higher than can be 

 used with ordinary tool steel, there would be much advantage in making 

 the hardening baths especially cold, and experience justified this as- 

 sumption. Sometimes the hardening bath was chilled with ice, but 

 usually it was used at room temperatures or even lukewarm. 



For rods of the dimensions of the test pieces I used, water, brine, 

 sulphuric acid and water, and the A" mixture seemed almost equally 

 effective in making the cast iron glass-hard, whether resistivity or 

 magnetic permeability of the hardened piece was used as the criterion. 



