February 26, 1920"! 



NATURE 



709 



RESEARCHES AT HIGH TEMPERATURES 

 AND PRESSURES. 



By the Hon. Sir Charles A. Parsons, K.C.B., 



F.R.S.' 



II. 



'T^ HE calories evolved in the combination of graphite 

 -'- and oxygen are about 05 per cent, less than those 

 evolved in the combination of diamond and oxygen, 

 indicating that graphite at ordinary temperature is, to 

 this extent, a stable state. The bulk-pressure which 

 has operated in some of the experiments would, how- 

 ever, seem to have been amply sulTicient to turn the 

 Oalance in favour of diamond instead of graphite. 

 The uncertainty, on the other hand, as to the com- 

 pressibilities and specific heats of the allotropic forms 

 of carbon under high pressures and at high tempera- 

 tures renders speculation of little value as to what may 

 occur at the melting point of carbon. All we know is 

 that, up to the pressures and temperatures reached in 

 our experiments, no indication of a change from 

 graphite to diamond has been produced. In one 

 experiment very intense heating was applied for five 

 seconds, but sufficient in amount to melt the graphite 

 core six times over, the only result being a slight 

 alteration in the structure of the graphite. The barrier 

 in this experiment was calcined magnesia, and the 

 hole in it was superficially converted to magnesium 

 carbide. It appeared, however, desirable further to 

 investigate the possibility of carbon losing its elec- 

 tr'ical conductivity when approaching its melting 

 point, as alleged by Ludwig and others, and of shunt- 

 ing the current from itself on to the contiguous molten 

 layers of the insulating barrier surrounding it. There 

 had been no indication of such a change having 

 occurred even momentarily ; it rather seemed that the 

 graphite core had been partially vaporised and con- 

 densed in the cooler parts of the charge. The experi- 

 ment was repeated with rods of iron and tungsten 

 embedded in the core, so that should the temperature 

 of volatilisation of the metals under a pressure of 

 15,000 atmospheres exceed that necessary to liquefy 

 carbon under the same pressure, the presence of these 

 metals might produce a different result. No change, 

 however, occurred. 



Note. — The temperatures at which carbon, iron, and 

 tungsten volatilise under a pressure of 15,000 atmo- 

 spheres are unknown, but they are probably much 

 higher than at atmospheric pressure. 



This experiment also tested iron as a solvent of 

 carbon and as a catalyst from diamond to graphite 

 under a pressure of 100 tons, and showed that under 

 this pressure that action was not reversed. 



Fig. 3 shows the container arranged for treating 

 powders by resistance heating with or without the 

 addition of liquids or gases. The electric current is 

 conveyed from the container to the upper end of the 

 conductor by a layer of graphite which rests on the 

 charge under treatment. The bottom end of the 

 conductor rests on or is spigoted into a cast-iron 

 block which rests on the bottom pole ; this block is 

 sometimes partially melted, but can be easily renewed. 



The container is charged by first stemming mag- 

 nesite powder by hand around the bottom pole-piece 

 and block ; then the charge is placed on the top and 

 pressed to 5 tons per square inch ; the top ram is 

 then removed, a hole drilled through the charsre, 

 and the conductor inserted. Liquids, if used, or 

 carbon dioxide snow may then be introduced ; lastly, 

 a layer of graphite is placed on the top, and the whole 

 pressed to the desired pressure for the experiment. 



1 Biscourse delivered at the Foyal 'Institution on Friday, January 23. 

 Continued from p. 681. 



NO. 2626, VOL. 104] 



In one experiment several pounds of carbon dioxide 

 snow were added to the charge, which consisted of 

 magnesia, and was so arranged that evaporation of 

 the heating carbon rod took place in an atmosphere 

 of carbon dioxide and carbon monoxide under a 

 gaseous pressure of 4400 atmospheres, the condensate 

 resulting being soft graphite. Upwards of two 

 hundred chemical reactions arranged to deposit carbon 

 were tested under high pressure and central heating. 

 After each experiment samples were taken from 

 various parts of the charge and carefully analysed 

 for diamond, the methods of the analyses generally 

 following those of Moissan and Crookes. On the 

 whole, there was no evidence that diamond had been 

 produced by any of the chemical reactions, some of 

 which were endothermic, such as carborundum and 

 sodium carbonate, which produced a grey solid which 

 detonated when struck with a hammer, and nearly 

 caused a serious accident. In one experiment the 

 charge was olivine and water ; when molten under 

 10 tons per square inch the pressure was suddenly 

 removed, and artificial pumice was formed by the 



PAPER Tube 



ASBESTOS n, 

 MtCA INSULATION 



^ MAGNESITE 



Rubber Cup 



PACKING 



VULCANIZeO 



riBRC Ring 



Fig. 3. — For powders with or without liquids and gases. 



expansion of water-vapour absorbed by the olivine 

 when molten. 



Having nearly reached the limits of steady pressure 

 obtainable in steel containers under a press, experi- 

 ments with impact pressures produced by steel bullets 

 were tried, which produced much higher instantaneous 

 pressures than are obtainable in any die. 



A rifle, o-303-in. bore, was arranged for with- 

 standing a charge of cordite 90 per cent, in excess of 

 the Service charge. The gun {Fig. 4) was fixed with 

 its muzzle 6 in. from a massive block of steel, in 

 which a hole 0-303 in. in diameter had been drilled 

 to a depth somewhat greater than the length of the 

 bullet, and in alignment with the bore of the gun; 

 cylindrical bullets of steel with a copper driving band 

 were chiefly used, shorter than the Service bullet and 

 about one-half the weight. The substance to be 

 compressed was placed either at the bottom of the 

 hole, when a conical-nosed bullet of mild steel was 

 used, or over the mouth of the hole, when a cupped- 

 nosed bullet of tool-steel was employed. About a 

 hundred experiments were made. 



