October 12, 1905] 



NA TURE 



595 



in culd, then in boiling acid. After this treatment the soft 

 graphite disappears, and most, if not all, the silicon com- 

 pounds have been destroyed. Hot sulphuric acid is again 

 applied to destroy the fluorides, and the residue, well 

 washed, is attacked with a mixture of the strongest nitric 

 acid and powdered potassium chlorate, kept warm — but 

 not above 60° C, to avoid explosions. This treatment 

 must be repeated six or eight times, when all the hard 

 graphite will gradually be dissolved, and little else left 

 but graphitic oxide, diamond, and the harder carbonado 

 and boart. The residue is fused for an hour in fluor- 

 hydrate of fluoride of potassium, then boiled out in water, 

 and again heated in sulphuric acid. The well washed 

 grains which resist this energetic treatment are dried, 

 carefully deposited on a slide, and examined under the 

 microscope. Along with numerous pieces of black diamond 

 are seen transparent colourless pieces, some amorphous, 

 others with a crystalline appearance. Although many 

 fragments of crystals occur, it is remarkable I have never 

 seen a complete crystal. All appear shattered, as if on 

 being liberated from the intense pressure under which they 

 were formed they burst asunder. I have singular evidence 

 of this phenomenon. A fine piece of artificial diamond, 

 carefully mounted by me on a microscopic slide, exploded 

 during the night and covered the slide with fragments. 

 .Moissan's crystals of artificial diamond sometimes broke a 

 few weeks after their preparation, and some of the 

 diamonds which cracked weeks or even months after their 

 preparation showed fissures covered with minute cubes. 

 This bursting paroxysm is not unknown at the Kimberley 

 mines. 



On the screen 1 will project photographs of artificial 

 diamonds manufactured in the manner described. So far, 

 these specimens are all microscopic. The largest artificial 

 diamond is less than one millimetre across. 



These laboratory diamonds burn in the air before the 

 bkiwpipe to carbonic acid. In lustre, crystalline form, 

 optical properties, density, and hardness, they are identical 

 with the natural stone. 



In several cases Moissan separated ten to fifteen micro- 

 scopic diamonds from a single ingot. The larger of these 

 are about 075 mm, long, the octahedra being 02 mm. 



Boiling ami Mclliiig Point of Carbon. 



On the average, the critical point of a substance is 

 15 times its absolute boiling point. Therefore the critical 

 point of carbon should be about 5800° Ab. But the 

 absolute critical temperature divided by the critical pressure 

 is for all the elements so far examined never less than 25, 

 this being about the value Sir James Dewar finds for 

 hydrogen. So that, accepting this, we get the maximum 

 critical pressure as follows, viz. 2320 atmospheres : — 

 5800° Ab./CrP = 2.5, or CrP = 58oo'' Ab./2-5, or 2320 

 atmospheres. 

 Carbon and arsenic are the only two elements that h.ive 

 a melting point above the boiling point ; and among com- 

 pounds carbonic acid and fluoride of silicium are the only 

 other bodies with similar properties. Now the melting 

 point of arsenic is about 1.2 times its absolute boiling 

 point. With carbonic acid and fluoride of silicium the 

 melting points are about i-i times their boiling points. 

 Applying these ratios to carbon, we find that its melting 

 point would be about 4400°. 



Therefore, assuming the following data 



Boiling point 3870° Ab. 



Mailing point ... ... 4400° 



Critical temperature ... ... ... 5800° 



Critical pressure .. ... .. 2320 Als. 



the Rankine or \'an der Waals formula calculated from 

 the boiling point and critical data would be as follows : — 



log. P = lo-i I — 39120/T, 

 and this gives for a temperature of 4400° Ab. a pressure 

 of 16.6 .\ts. as the melting-point pressure. Similar rough 

 estimates obtained by means of this formula suggest that 

 above a temperature of 5800° Ab. no amount ofpressure 

 will cause carbon vapour to assume liquid form, whilst at 

 4400° Ab. a pressure of above 17 atmospheres would suffice 

 to liquefy some of it. Between these extremes the curve 



NO. 1876, VOL. 72] 



of vapour pressure is assumed to be logarithmic, as re- 

 presented in the accompanying diagram. The constant 

 39120 which occurs in the logarithmic formula enables 

 us to calculate the latent heat of evaporation. If we 

 assume the vapour density to be normal, or the molecule 

 in vapour as C,, then the heat of volatilisation of 12 grms. 

 of carbon would be 90,000 calories ; or, if the vapour is a 

 condensed molecule like Cj, then the 12 grms. would need 

 30,000 calories. In the latter case the evaporation of 

 I grm. of carbon would require 2500 calories, whereas a 

 substance like zinc needs only about 400 calories. 



A New Fonnation of Diamond. 



I have long speculated as to the possibility of obtaining 

 artificially such pressures and temperatures as would fulfil 

 the above conditions. In their researches on the gases 

 from fired gunpowder and cordite. Sir Frederick Abel and 

 .Sir Andrew Noble obtained in closed steel cylinders 

 pressures as great as 95 tons to the square inch, and 

 temperatures as high as 4000° C. According to a paper 

 recently communicated to the Royal Society, Sir Andrew 

 Noble, exploding cordite in closed vessels, has obtained 

 a pressure of 8000 atmospheres, or 50 tons per square inch, 

 with a temperature reaching in all probability 5400° .'\b. 



Here, then, we have conditions favourable for the lique- 

 faction of carbon, and were the time of explosion sufficient 

 to allow the reactions to take place, we should certainly 

 expect to get the liquid carbon to solidify in the crystalline 

 state. ^ 



By the kindness of Sir Andrew Noble, I have been 

 enabled to work upon some of the residues obtained in 

 closed vessels after explosions, and I have submitted them 

 to the same treatment that the granulated iron had gone 

 through. After weeks of patient toil I removed the 

 amorphous carbon, the graphite, the silica,' and other 

 constituents of the ash of cordite, and obtained a residue 

 among viihich, under the microscope, crystalline particles 

 could be distinguished. Some of these particles, from 

 their crystalline appearance and double refraction, were 

 silicon carbide : others were probably diamonds. The 

 whole residue was dried and fused at a good red heat in 

 an excess of potassium bifluoride, to which was added 

 during fusion 5 per cent, of nitre. (Previous experiments 

 had shown me that this mixture readily attacked and 

 dissolved silicon carbide ; unfortunately it also attacks 

 diamond to a slight degree.) The residue, after thorough 

 washing and then heating in fuming sulphuric acid, was 

 washed, dried, and the largest crystalline particles picked 

 out and mounted. .Ml the operations of washing and acid 

 treatment were performed in a large platinum crucible 

 by decantation (except the preliminary attack with nitric 

 acid and potassium chlorate, when a hard glass vessel was 

 used) ; the final result was washed into ashallow watch- 

 glass, and the selection made under the microscope. 



I project on the screen a few photographs of these 

 crystals. From the treatment they have undergone, 

 chemists will agree with me that diamonds only could 

 stand such an ordeal; on submitting them to skilled 

 crystallographic authorities my opinion is confirmed. 

 -Speaking of the one before you' (303), Prof. Bonnev calls 

 it "a diamond showing octahedral planes with dark 

 boundaries due to high refracting index." After careful 

 examination. Prof. Miers writes of the same crystal 

 diamond : — " I think one may safely say that the position 

 and angles of its faces, and of its cleavages, the absence 

 of birefringence, and the high refractive index, are all 

 compatible with the properties of the diamond crystallising 

 in the form of an octahedron. Others of the remaining 

 crystals, which show a similar high refractive index, 

 appeared to me to present the same features." 



1 Sir James Dewar, in a Kriday evening discourse at the Roval Ins'i- 

 lutioi, i83o, showed an experiment proving tliat the temperature of the 

 inienor of acarbon tube heated bv an outside electric arc was higher than 

 that of the oxyhydroeen flame. He placed afew small crystals of diamond 

 in the carbon tube, .ind, maintaining a current of hydrogen 10 prevent oxi- 

 dation, raised the temperature of the tube in an electric furnace to that of 

 the arc. In a few minutes the diamond w.is transformed into graphite. At 

 first sight this would seem to show that diamond cannot be formed at tem- 

 peratures above that of the arc- It is probable, however, for reasons given 

 above, that at exceedingly high pressures the result would be different 



- The silica was in the form of spheres, perfectly shaped and Iransparent 

 mostly colourless, but among them several r,f a ruby colour. When 5 per 

 cent, of silica was added to cordite, the residue of the closed vessel explosion 

 contained ,1 much larger quantity of these spheres. 



