76 



THE HON. SIR CHAKLES ALGERNON PARSONS: EXPERIMENTS ON 



on the breech screw appeared not to have exceeded this pressure. Highly carburized 

 iron, therefore, does not expand with any considerable force on setting. 



The reason why a lump of cast iron thrown into a ladle of molten metal first sinks 

 to the bottom and soon rises and floats on the surface is probably that cast iron is 

 about seven times stronger in compression than in tension. Therefore when a 

 sufficiently thick layer of the cold metal has been heated the interior is torn asunder 

 by the expansion of the outer skin, and the specific gravity of the whole mass is 

 diminished. (See Mr. WRIGHTSON'S paper " On Iron and Steel at High Temperatures," 

 with discussion, ' Journal of the Iron and Steel Institute,' No. 1 for 1880.) 



We may therefore safely conclude that when iron is suddenly cooled, the only 

 compressive bulk pressure that is brought to bear on the interior is that arising from 



Fig. 9. 



the contraction of the outer layers after setting, and with highly carburized iron 

 this can only be small because of the low tensile strength of the metal. 



Gases Ejected from Cast Iron on Setting. 



As bearing upon the question of the possibility of the occluded gases playing a 

 part, MOISSAN was the first to observe that spherules or small spheres of iron with 

 cracks and geodes never contained diamond. We have made experiments by pouring 

 highly carburized iron, alloys and mixtures on to iron plates, the cooling taking place 

 from one side only, and under such conditions no diamond results ; in fact it only 

 occurs when the ingot or spherule is cooled on all sides nearly simultaneously, so that 

 an envelope of cold metal is formed all over before the centre sets. 



