August 5, 1897 J 



NATURE 



327 



porous masses, and in massive black pebbles, sometimes weigh- 

 ing a couple or more ounces. 



Diamonds vary considerably in hardness, and even different 

 parts of the same crystal are decidedly different in their resist- 

 ance to cutting and grinding. The famous Koh-i-noor, when 

 cut into its present form, showed a notable variation in hardness. 

 In cutting one of the facets near a yellow flaw, the crystal 

 became harder and harder the further it was cut into, until, 

 after working the mill for six hours at the usual speed of 2400 

 revolutions a minute, little impression was made. The speed 

 was accordingly increased to more than 3000, when the work 

 slowly proceeded. Other portions of the stone were found to 

 be comparatively soft, and became harder as the outside was 

 cut away. 



Beautifully white diamonds have been found at Inverel, New 

 South Wales, and from the rich yield of the mine and the white 

 colour of the stones, great things were expected. A parcel 

 of many hundred carats came to England, when it was found 

 they were so hard as to be practically unworkable as gems, 

 and I believe they were ultimately sold for rock-boring 

 purposes. 



I will illustrate the intense hardness of the diamond by an 

 experiment. I place a diamond on the flattened apex of a 

 conical block of steel, and on the diamond I bring down a 

 second cone of steel. With the electric lantern I will project 

 an image of the diamond and steel faces on the screen, and 

 force them together by hydraulic power. Unless I happen to 

 have selected a diamond with a flaw, I shall squeeze the stone 

 right into the steel blocks without injuring it in the slightest 

 degree. 



But it is not the hardness of the diamond so much as its 

 optical qualities that make it so highly prized. It is one of the 

 most refracting substances in nature, and it also has the highest 

 reflecting properties. In the cutting of diamonds advantage is 

 taken of these qualities. When cut as a brilliant the facets on 

 the lower side are inclined so that light falls on them at an 

 angle of 24" 13', at which angle all the incident light is totally 

 reflected. A well-cut diamond should appear opaque by trans- 

 mitted light, except at a small spot in the middle where the 

 table and culet are opposite. All the light falling on the front 

 of the stone is reflected from the facets, and the light passing 

 into the diamond is reflected from the interior surfaces and 

 refracted into colours when it passes out into the air, giving rise 

 to the lightnings and coruscations for which the diamond is 

 supreme above all other gems. 



The following table gives the refractive indices of diamonds 

 and other bodies : — 



Refractive Indices for the D Line. 



Chromate of lead 



Diamond 



Phosphorus 



Sulphur 



Ruby 



Thallium glass 

 Iceland spar 

 Topaz 



Beryl 



Emerald 



Flint glass 



Quartz 



Canada balsam 



Crown glass 



Fluor-spar 



Ice 



2 -50-2 -97 

 2-47-275 



2-22 

 212 

 178 



I -65 

 I 61 

 I 60 

 1-59 

 1-58 

 I 55 

 1-53 

 I "53 

 1-44 



1-31 



(U-I 



According to Dr. Gladstone, the specific refractive energy, 

 , will be for the D line 0-404, and the refraction equivalent, 



P ^S will be 4 82. 

 a 



After exposure for some time to the sun many diamonds glow 

 in a dark room. Some diamonds are fluorescent, appearing 

 milky in sunlight. In a vacuum, exposed to a high-tension 

 current of electricity, diamonds phosphoresce of different 

 colours, most South African diamonds shining with a bluish 

 light. Diamonds from other localities emit bright blue, apricot, 

 pale blue, red, yellowish green, orange, and pale green light. 

 The most phosphorescent diamonds are those which are 



\0. 1449. VOL. 56] 



fluorescent in the sun. One beautiful green diamond in my 

 collection, when phosphorescing in a good vacuum, gives almost 

 as much light as a candle, and you can easily read by its rays. 

 The light is pale green, tending to white. 



Conversion of Diamond into Graphite. 



I will now draw your attention to a strange property of the 

 diamond, which at first sight might seem to argue against the 

 great permanence and unalterability of this stone. It has been 

 ascertained that the cause of phosphorescence is in some way con- 

 nected with the hammering of the gaseous molecules, violently 

 driven from the negative pole, on to the surface of the body 

 under examination ; and so great is the energy of the bombard- 

 ment, that impinging on a piece of platinum, or even iridium, 

 the metal will actually melt. When the diamond is thus bom- 

 barded in a radiant matter tube the result is startling. It not 

 only phosphoresces, but assumes a brown colour, and when the 

 action is long continued becomes almost black, 



I will project a diamond on the screen and bombard it with 

 radiant matter before your eyes. Some diamonds visibly darken 

 in a few minutes, while others, more leisurely in their ways, 

 require an hour. 



This blackening is only superficial, but no ordinary means of 

 cleaning will remove the discolouration. Ordinary oxidising 

 reagents have little or no effect in restoring the colour. The 

 black stain on the diamond is due to a form of graphite which 

 is very resistant to oxidation. It is not necessary to expose the 

 diamond in a vacuum to electrical excitement in order to pro- 

 duce a change. 



I have already signified that there are various degrees of 

 refractoriness to chemical reagents among the different forms of 

 graphite. Some dissolve in strong nitric acid ; other forms of 

 graphite require a mixture of highly concentrated nitric acid 

 and potassium chlorate to attack them, and even with this in- 

 tensely powerful agent some graphites resist longer than others. 

 M. Moissan has shown that the power of resistance to nitric 

 acid and potassium chlorate is in proportion to the temperature 

 at which the graphite was formed, and with tolerable certainty 

 we can estimate this temperature by the resistance of the speci- 

 men of graphite to this reagent. 



The superficial dark coating on a diamond after exposure to 

 molecular bombardment I have proved to be graphite [Chemical 

 News, vol. Ixxiv., p. 39, July 1896), and M. Moissan {Comptes 

 rendus, cxxiv. p. 653) has shown that this graphite, on account 

 of its great resistance to oxidising reagents, cannot have been 

 formed at a lower temperature than 3600° C. 



It is therefore manifest that the bombarding molecules, carry- 

 ing with them an electric charge, and striking the diamond with 

 enormous velocity, raise the superficial layer to the temperature 

 of the electric arc, and turn it into graphite, whilst the mass of 

 diamond and its conductivity to heat are sufficient to keep down 

 the general temperature to such a point that the tube appears 

 scarcely more than warm to the touch. 



A similar action occurs with silver, the superficial layers of 

 which can be raised to a red heat without the whole mass be- 

 coming more than warm {Proc. Roy. Soc, vol. 1. p. 99, June 

 1891). 



This conversion of diamond into graphite is, I believe, a pure 

 effect of heat. In 1880 [Proceedings of the Royal Institution) 

 Friday Evening Meeting, January 16, 1880) Prof. Dewar, in 

 this theatre, placed a crystal of diamond in a carbon tube, 

 through which a current of hydrogen was maintained. The 

 tube was heated from the outside by an electric arc, and in a few 

 minutes the diamond was converted into graphite. I will now 

 show you that a clear crystal of diamond, heated in the electric 

 arc (temperature 3600° C.) is converted into graphite, and this 

 graphite is most refractory. 



The diamond is remarkable in another respect. It is ex- 

 tremely transparent to the Rontgen rays, whereas highly 

 refracting glass, used in imitation diamonds, is almost perfectly 

 opaque to the rays, I exposed over a photographic plate to the 

 X-rays for a few seconds the large Delhi diamond, of a fine pink 

 colour, weighing 31^ carats, a black diamond weighing 23 carats, 

 together with an imitation in glass of the pink diamond lent me 

 by Mr. Streeter ; also a flat triangular crystal of diamond of 

 pure water, and a piece of glass of the same shape and size. On 

 development, the impression where the diamond obscured the 

 rays was found to be strong, showing that most rays passed 

 through, while the glass was practically opaque. By this means 

 imitation diamonds and some other false gems can readily be 



