THE ARTIFICIAL PRODUCTION OF DIAMOND. 8'J 



it was noticed that in the water-cooled areas of the quartz tube a lustrous black ring 

 had formed. On being strongly heated, some of this, evidently carbon, burnt off, 

 leaving a white film, presumably silica. This seems to show that a volatile silico- 

 organic compound, containing carbon, hydrogen, and silicon, was evolved from the 

 iron on heating." 



It would appear from our experiments that probably a ferro-silicon carbouyl is 

 given off from the iron, for, as has been said, we observed a corrosive action on 

 carborundum by the gas evolved from iron borings at red heat under a high vacuum, 

 and the same action was produced by gaseous ferro-carbonyl, and also by carbon 

 monoxide, previously passed over molten iron sulphide at atmospheric pressure. 



Let us consider what happens in an ingot or spherule when rapidly cooled 

 simultaneously on all sides. It is first surrounded by a thin coat of solidified metal 

 which, below 600 C., is impervious to gases. As the coat thickens layer within 

 layer, more and more gas is ejected by the solidifying metal, and its semi-solidified 

 centre, still pervious to gas, receives the charge. As this process progresses the 

 pressure may rise higher and higher, though there may be a limit to the pressure 

 against which the metal is able to eject gas when setting. All we, however, know is, 

 that the mechanical strength of" the ingot or spherule places a limit of about 7000 

 atmospheres on the gaseous pressure, and, as we have already mentioned in the case 

 of some iron alloys, most. of the spherules are split or shredded, with an appearance 

 consistent with this view. 



CROOKES' microscopical examination of diamonds with polarized light supports this 

 view. In his lecture at Kimberley, in 1905, lie states: "I have examined many 

 hundred diamond crystals under polarized light, and with few exceptions all show 

 the presence of internal tension. 



" On rotating the polarizer, the black cross most frequently seen revolves round a 

 particular point in the inside of the crystal; on examining this point with a high 

 power we sometimes see a slight flaw, more rarely a minute cavity. The cavity is 

 filled with gas at enormous pressure, and the strain is set up in the stone by the 

 effort of the gas to escape." 



It seems therefore probable, or indeed almost certain, from the accumulated 

 evidence, that the chief function of quick cooling in tl'ie production of diamond in an 

 ingot or spherule is to bottle up and concentrate into local spots the gases occluded 

 in the metal which, under slow cooling, would partially escape and the remainder 

 become evenly distributed throughout the mass. 



As to the condition in which the gases exist within the iron at temperatures above 

 500 C. little is known, though at 200 C. and at 180 atmospheres MOND has shown 

 that iron penta-carbonyl is formed. The intimate contact between the occluded 

 gases and other elements, metals or carbides, must favour complex interactions as 

 cooling takes place. Such actions might be concentrated by the heat flow across the 

 metal on quick cooling. 



