202 



PROCEEDINGS OF THE AMERICAN ACADEMY. 



TABLE XI. 



If the material used in these experiments may be considered typical 

 of the so-called " pure cast iron " from good foundries, it appears, then, 

 that an annealed casting may have at room temperatures a resistivity, 

 referred to the centimeter cube, as low as 0.000073 or as high as 

 0.000104 ; that it is always possible to make the specimen glass-hard 

 throughout by heating it to a temperature a little below the melting 

 point and chilling it in a suitable bath ; and that the process, as Barus 

 and Strouhal showed, is always accompanied by an increase in resis- 

 tivity. This increase is sometimes only about ten per cent of the 

 original value, though it is oftener nearly twenty-five per cent and may 

 rise somewhat higher. Only one kind of iron that I used resisted 

 successfully a noticeable relaxation of temper in the hardened pieces 

 by prolonged boiling in water. Of two pieces of iron from the same 

 pouring, which have equal resistivities when first annealed, that one 

 has the higher resistivity, after both have been hardened, which has 

 the lower magnetic permeability. Tests of mechanical hardness are 



