326 



NATURE 



[August 5, 1897 



Physics of the Diamond. 



The specific gravity of the diamond is from 3*514 to 3'5i8. 



For comparison, I give in tabular form the specific gravities 



of the different varieties of carbon : — 



Amorphous carbon i'45 to 1*70 



Graphite 211 ,, 3-0 



Hard gas coke 2'356 



Boart 3-47 ,, 3'49 



Carbonado ... .... ... 3 '50 



Diamond • 3'5i4 jj 3'Si8 



Fig. I.— Triangular markings on natural face of a diamond. 



The following table gives the specific gravities of the 

 minerals found on the sorting tables. I have also included 

 the specific gravities of two useful liquids : — 



Specific gravity. 



Hard graphite 25 



Quartzite and granite 2 '6 



Beryl 27 



Mica 2*8 



Hornblende ... 3*0 



Methylene Iodide 3-3 



Diamond 3*5 



Thallium Lead Acetate 3'6 



Garnet 37 



Corundum 39 



Zircon ... ... .. ... ... 4*4 



Barytes 45 



Chrome and titanic iron ore 47 



Magnetite... ... ... ... ... 5'o 



Fig. 2. — Artificial markings on face of a diamond, produced 

 by partial combustion. 



This table shows that if I throw the whole mixture of 

 minerals into methylene iodide, the hornblende and all above 

 that mineral will rise to the surface ; while the diamond and all 



NO. 1449, VOL. 56] 



minerals below will sink to the bottom. If I now take these 

 heavy minerals, and throw them into thallium lead acetate, they 

 will all sink, except the diamond, which floats and can be 

 skimmed off. 



The diamond belongs to the isometric system of crystallo- 

 graphy. It frequently occurs with curved faces and edges. 

 Twin crystals (niacles) are not uncommon. Having no double 

 refraction it should not act on polarised light. But, as is well 

 known, if a transparent body which does not so act is submitted 

 to strain of an irregular character ir 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. 

 Under polarised light I have examined many hundred diamond 



Fig. 3. — Natural crystals of diamond. 



crystals, and with few exceptions all show the presence oj 

 internal tension. On rotating the polariser, the black cross, 

 which is most frequently seen, revolves round a particular point 

 in the inside of the crystal, and on examining this point with a 

 high power, we see sometimes a slight flaw, more rarely a 

 minute cavity. The cavity is filled with gas at an enormous 

 pressure, and the strain is set up in the stone by the effort of the 

 gas to escape. 



It is not uncommon for a diamond to explode soon after it 

 reaches the surface, and some have been known to burst in the 

 pockets of the miners or when held in the warm hand. Large 

 crystals are more liable to burst than smaller pieces. Valuable 

 stones have been destroyed in this way, and it is whispered that 

 cunning dealers are not averse to allowing responsible clients to 

 handle or carry in their warm pockets large crystals fresh fiom 

 the mine. By way of safeguard against explosion, some dealers 

 embed large diamonds in raw potato to ensure safe transit to 

 England. 



In the substance of many diamonds we find enclosed black 

 uncrystallised particles of graphite. There also occur what 

 may be considered intermediate forms between the well- 

 cr}stallised diamond and graphite. These are " boart " and 

 "carbonado." Boart is an imperfectly crystallised diamond, 

 having no clear portions ; therefore it is useless for gems. Boart 



Fig. 4. — Natural crystals of diamond. 



is frequently found in spherical globules, and may be of all 

 colours. It is so.hard that it is used in rock-drilling, and when 

 crushed it is employed for cutting and polishing other stones. 

 Carbonado is the Brazilian term for a still less perfectly 

 crystallised form of carbon. It is equally hard, and occurs in 



