BRIDGMAN. — ACTION OF MERCURY ON STEEL. 337 



gamation of these cylinders we have in the first place the strong natural 

 ailinity between iron anil mercury. This is prevented from coming into 

 play under ordinary circumstances by a thin layer of dirt on the surface. 

 But it seems reasonable to suppose that amalgamation will start in the 

 mass of the metal if the mercury can be once forced into the pores, 

 since under these circumstances the iron and mercury coming into con- 

 tact with each other would be clean and the natural chemical affinity 

 come into play. The argument consists in showing that in those cases 

 where amalgamation took place the conditions were such as to favor the 

 introduction of mercury into the pores of the steel, even if the surface 

 were not amalgamated. Then after amalgamation is once started in 

 the mass of the metal it is assisted in the rapidity of its growth from 

 the amalgamated region by the action of hydrostatic pressure. 



This discussion demands a slight consideration of the nature of the 

 strain in a hollow cylinder exposed to internal fluid pressure. The 

 stresses in the metal of the cylinder consist of a pressure (negative) 

 across planes perpendicular to the radii, and a tension (positive) across 

 radial planes. These stresses are greatest arithmetically at the interior 

 surface, but the algebraic sum is constant throughout the mass of the 

 cylinder. This has as a consequence that the volume strain in the 

 cylinder is a dilation and is everywhere constant, so that the pores are 

 opened up by the action of the stress and the entrance of mercury 

 facilitated. This holds while the strain remains elastic. But when 

 the internal pressure exceeds a certain value so that at the inner sur- 

 face the algebraic difference between the radial pressure and the cir- 

 cumferential tension exceeds a critical value depending on the elastic 

 limit, the strain becomes inelastic, the tension changes over to a pressure 

 so that both principle stresses become compressions, the volume strain 

 changes from a dilatation to a compression, and the pores close up. So 

 that with a steel of low elastic limit the type of stress may change, 

 giving a volume compression, at a lower value of the fluid pressure and 

 therefore at a smaller preliminary volume dilatation than in a steel of 

 higher elastic limit. This view as to the nature of the stress in a thick 

 cylinder stressed beyond the elastic limit is supported by many other 

 experiments on the bursting of thick cylinders. An account of these 

 experiments will be published in another paper. 



The difference found in the rupture points between soft and hardened 

 steel is to be ascribed to two causes. One is the greater intrinsic ease 

 of driving amalgamation through a mass of hardened steel by hydro- 

 static pressure. This was proved by the experiments on the broken 

 amalgamated test pieces. It may be due in part to chemical difference 

 between the hard and soft steel, but is almost certainly also due in part 



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