ipis.] IN RECENTLY HARDENED STEEL. 169 



Having no more of the carbon steel used in the first experiment, 

 I procured another half-inch round bar of the same brand, though 

 slightly difiierent in composition as the analyses show. With a piece 

 of this bar two and a half inches long I made a careful determina- 

 tion of its specific gravity under the conditions, and with the results, 

 shown in the following table. 



Table i. 



Specific Gravity Analysis of Steel 



Commercial Condition 7.8507 Phosphorus 0.015 



After Hardening 7.8127 Sulphur 0.021 



After Tempering to Light Blue 7.8350 Silicon 0.16 



After Annealing 7.8529 Manganese 0.33 



Carbon i .07 



The difiference in density and volume between the hardened and 

 annealed conditions is fully a half per cent., which is much more 

 than I expected to find ; and nearly half of the total shrinkage was 

 brought about by the very moderate heating necessary for " temper- 

 ing to light blue." The annealing was very thorough, and, as the 

 figures show, was more complete than in the annealed " commercial 

 condition." 



The shrinkage incident to tempering was large enought to en- 

 courage the hope that if any spontaneous shrinking, at room tem- 

 perature, occurs during the generation of heat which follows harden- 

 ing, it might be detected and measured. For this purpose the ap- 

 paratus shown in Fig. 4 was designed and constructed. 



In Fig. 4 G and H are two vertical steel rods three feet long 

 and one millimeter in diameter. They are hung from a common 

 rigid metal support /, and at their lower ends carry parallel brass 

 bars G' , H' which move with perfect freedom, yet in close contact, 

 between guides K, K. The brass bars are accurately machined, 

 and their front edges are polished. The rod G, whose function is 

 purely comparative, is kept under moderate and constant tension by 

 the long spiral spring L; while the rod H carries a four pound 

 weight M. An enlarged sectional diagram at the right shows the 

 method employed in mounting each steel rod. Each end of the rod 

 passes through, and is soldered into, a brass head having a hemi- 



