76 
THE HON. SIR CHARLES 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 l)rought 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. 
