October 12, 1905 J 



NA TURE 



597 



previous time had been exposed to incipient combustion. 

 Rose also noted that striations appeared on the surfaces 

 of diamonds burnt before the blowpipe. 



1 have tried many times to imitate these markings by 

 partial combustion of clear crystals of diamond, but have 

 not succeeded in reproducing triangles of such beauty as 

 you see formed by nature. According to the crystalline 

 face exposed to incipient combustion the etchings are 

 triangular or cubical, and sometimes intermediate between 

 the two. I throw on the screen magnified photographs of 

 these etchings, and you will observe that while the 

 triangular or box-like tendency is very apparent, there is 

 an absence of regularity and sharpness. 



The artificial markings are closer massed, looking as if 

 the diamond during combustion had been dissected into 

 triangular and rectangular flakes, while the markings 

 natural to crystals appear as if produced by the crystal- 

 lising force as they were being built up. 



Certain artificial diamonds present the appearance of an 

 elongated drop. I have seen diamonds which have exactly 

 the appearance of drops of liquid separated in a pasty con- 

 dition and crystallised on cooling. Diamonds are some- 

 times found with little appearance of crystallisation, but 

 with rounded forms similar to those which a liquid might 

 assume if kept in the midst of another liquid with which 

 it would not mix. Other drops of liquid carbon retained 

 for sufficient time above their melting point would coalesce 

 with adjacent drops, and on slow cooling would separate 

 in the form of large perfect crystals. Two drops, joining 

 after incipient crystallisation, might assume the not un- 

 common form of interpenetrating twin crystals. Illus- 

 trations of all these caprices are here to-night. 



Again, diamond crystals are generally perfect on all 

 sides. They show no irregular side or face by which 

 they were attached to a support, as do artificial crystals 

 of chemical salts ; another proof that the diamond must 

 have crystallised from a dense liquid. 



Having no double refraction, the diamond should not 

 act on polarised light. But, as is well known, if a trans- 

 parent body which does not so act is submitted to strain 

 of an irregular character it becomes doubly refracting, and 

 in the polariscope reveals the existence of the strain by 

 brilliant colours arranged in a more or less defined pattern 

 according to the state of tension in which the crystal 

 exists. I have examined many hundred diamond crystals 

 under polarised light, and with few exceptions all show 

 the presence of internal tension. I will project some 

 diamonds on the screen by means of the polarising micro- 

 scope, and you will see by the colours how great is the 

 strain to which some of them are exposed. On rotating 

 the polariser, 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. I 

 have already told you that the great Cullinan diamond 

 by this means reveals a state of internal stress and strain. 



It is not uncommon for a diamond to explode soon after 

 it reaches the surface ; some have been known to burst in 

 the pockets of the miners or when held in the warm hand, 

 and the loss is the greater because large stones are more 

 liable to explode or fly in pieces than small ones. 

 Valuable stones have been destroyed in this way, and it 

 is whispered that cunning dealers are not averse to allow- 

 ing responsible clients to handle or carry in their warm 

 pockets large crystals fresh from the mine. By way of 

 safeguard against explosion, some dealers imbed large 

 diamonds in raw potato to ensure safe transit to England. 



The anomalous action which many diamonds exert on 

 polarised light is not such as can be induced by heat, but 

 it can easily be conferred on diamonds by pressure, show- 

 ing that the strain has not been produced by sudden cool- 

 ing, but by sudden lowering of pressure. 



The illustration of this peculiarity is not only difficult, 

 but sometimes exceedingly costly — difficult because it is 

 necessary to arrange for projecting on the screen the image 

 of a diamond crystal between the jaws of a hydraulic press, 

 the illuminating light having to pass through delicate 

 optical polarising apparatus — and costly because only 

 perfect, clear crystals can be used, and crystals of this 



NO. 1876, VOL. 72] 



character sometimes fly to pieces as the pressure rises. 

 No colour as yet is seen on the screen, the crystal not 

 being birefringent. A movement of the handle of the 

 press, however, gives the crystal a pinch, instantly re- 

 sponded to by the colours on the screen, showing the pro- 

 duction of double refraction. Another movement of the 

 handle brightens the colours ; a third may strain the 

 crystal beyond its power of resistance, so I refrain. 



Hardness. 



Diamonds vary considerably in hardness, and even 

 different parts of the same crystal differ in their resistance 

 to cutting and grinding. 



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. In the first parcel which came to England the 

 stones were found to be so much harder than South African 

 diamonds that it was at first feared they would be useless 

 except for rock-boring purposes. The difficulty of cutting 

 them disappeared with improved appliances, and they now 

 are highly prized. 



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, and after working the 

 mill for six hours at the usual speed of 2400 revolutions a 

 minute, little impression was made. The speed was in- 

 creased to more than 3000, when the work slowly pro- 

 ceeded. Other portions of the stone were found to be 

 comparatively soft, and hardened as the outside was cut 

 away. 



I can illustrate the intense hardness of the diamond by 

 experiment. On the flattened apex of a conical block of 

 steel I place a diamond, and upon it I bring down a second 

 cone of steel. With the lamp I project an image of the 

 diamond and steel faces on the screen, and force Ihem 

 together by hydraulic power. I can squeeze the stone into 

 the steel blocks without injuring it in the slightest degree. 



The pressure gauge shows 60 atmospheres, and the 

 piston being 32 inches diameter, the absolute pressure is 

 3- 16 tons, equivalent on a diamond of 12 square mm. 

 surface to 170 tons per square inch of diamond. 



Although not directly bearing on the subject, I will 

 introduce the only serious rival of the diamond as regards 

 hardness. It is the metal tantalum, a fine specimen of 

 which I owe to Messrs. Siemens Brothers. A hole had to 

 be bored through a plate of this metal, and a diamond 

 drill was used revolving at the rate of 5000 revolutions per 

 minute. This whirling force was continued ceaselessly for 

 three days and nights, when it was found that on'ly a 

 small depression ^ mm. deep had been drilled, and it was 

 a moot point which had suffered most damage, the diamond 

 or the tantalum.' In another respect tantalum is likely 

 to rival graphitic carbon, as it has rivalled adamantine 

 carbon. Its thin wire is extensively used for filaments 

 of_ incandescent electric lamps ; it shows a much higher 

 efficiency than does the old carbon filament. The melting 

 point of tantalum is about 2300° C, a temperature seldom 

 or never reached in an ordinary lamp. 



Refractivity. 

 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 

 brilliant should appear opaque by transmitted 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 pass- 

 ing 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, the effulgence, and 

 coruscations for which the diamond is supreme above all 

 other gems. 



In vain I have searched for a liquid of the same refrac- 

 1 W. von Bolton Zcitsiltr. Elcktrocluni , ii., 45-5r, January 20, 1905 



