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PROCEEDINGS OF THE AMERICAN ACADEMY. 



to the greater porosity of the hardened steel. It is well known that 

 steel decreases in density on hardening. The other cause is the higher 

 elastic limit of the hardened steel and the consequent wider opening 

 of the pores before non-elastic closing sets in. For facility of compari- 

 son the elastic limits of the cylinders of the various grades of steel are 

 given (see Table III), on the usual assumption that the elastic limit is 

 determined by the maximum stretch at the interior. This assumption 

 is probably not very near the truth, so the results can be expected to 

 show only qualitative agreement. This table does not apply to the 



TABLE III. 



cylinders in which the bottoms were blown out, as here the stress is of 

 a different nature and the above values of the elastic limit do not hold. 

 It appears from the table that only those cylinders were broken in which 

 the stress was below the elastic limit. That the cylinders of bessemer 

 and soft nickel steel were not broken may be explained by supposing 

 that the pressure required to open the pores wide enough to force the 

 mercury in is higher than the elastic limit. 



It remains to explain the form of the amalgamated region found in 

 the hardened nickel steel cylinders; i. e., bands on either side of the 

 crack. It seems certain that slight inequalities in the structure of the 

 steel will greatly affect the ease of amalgamation. This seems proved 

 by the splotches of amalgamation found throughout the broken test 

 pieces amalgamated by hydrostatic pressure and throughout the 

 broken cylinders. Conceive, then, of a hollow cylinder filled with 

 mercury subjected to pressure. The pressure expands the pores of the 

 metal, and in consequence of the high mobility of the mercury molecule 

 the mercury is forced into these pores through the layer of dirt lining 

 the hole. At certain places where the metal is more susceptible the 



