598 



NATURE 



[October 12, 1905 



tion as diamond. Such a liquid would be invaluable to the 

 merchant, as on immersing a stone the clear body would 

 absolutely disappear, leaving in all their ugliness the flaws 

 and black specks so frequently seen even in the best stones. 

 .•\rguing from theoretical considerations connected with 

 the specific refractive energy of diamond, and employing 

 Lorentz's expression for refraction — 





in which /i = refractive index, ^ — i = refractive energy, 

 d = density, and P = molecular weight, Brtihl has shown 

 that diamond is perfectly normal in its optical properties, 

 and has an atomic refraction =5. He has put forward 

 the speculation that the diamond may be the last member 

 of the parafl^n series of which marsh-gas is the first. 



" Now we can imagine," says Briihl,' " why the 

 diamond, i.e. pure crystallised carbon, is optically normal. 

 We obtain an idea of the mineral's chemical constitution, 

 and of the way in which the atoms of carbon are perhaps 

 combined in the sparkling gem. The diamond cannot 

 possibly contain any double bonds. Imagine, however, at 

 each of the six corners of a regular octahedron, a single 

 molecule of marsh-gas, CH,, i.e. altogether C^H,,, and 

 then imagine all the 24 hydrogen atoms successivelv 

 removed, so that each carbon atom is connected with each 

 of its neighbours only by a single bond, and thus all six 

 atoms of carbon are united together in a single whole. 

 Then you obtain, as the most simple representation of the 

 molecule of the diamond, a regular octahedron, with one 

 atom of carbon at each of its six corners, whilst the edges 

 represent the mutual bonds : — 



Several simple molecules of this kind may be combined 

 into one crystallised particle of the spectrochemicallv 

 normal diamond." 



Absorption .'>pi'ctrum of Diamond. 

 On passing a ray of light through a diamond and ex- 

 amining it in a spectroscope, B. Walter has found in all 

 colourless brilliants of more than one carat in weight an 

 absorption band at wave-length 4155 (violet). He ascribes 

 this band to an impurity, and suggests it may possiblv be 

 due to samarium. Three other fainter lines were detected 

 in the ultra-violet by means of photography. 



Phosphorescence of DiaiiioiiJ. 



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, e.xposed 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 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. But the time has hardly 

 come when diamonds can be used as domestic illuminants ! 

 The emitted light is pale green, tending to white, and in 

 its spectrum, when strong, can be seen bright lines, one 

 at about \ 5370 in the green, one at \ 5130 in the greenish 

 blue, and one at A. 5030 in the blue. 



.After many years' bombardment in a vacuum tube this 

 diamond grew very dark, almost black, on the surface. 

 Heating in a mixture of nitric acid and potassium chlorate 

 scarcely changed the colour. The action of heat was then 

 tried, and on slowly heating to about 500° C. the dark 

 1 Proceeding.': of the Roynl Institution, May 26, 1905. 



NO. 1876, VOL. 72] 



colour entirely disappeared, and the original milky green 

 appearance was restored. .Although I watched narrowly 

 I could see no trace of phosphorescence during the heating. 



Diamonds which phosphoresce red generally show the 

 yellow sodium line superposing on a continuous spectrum. 

 In one Brazilian diamond phosphorescing a reddish yellow- 

 colour, I detected the citron line characteristic of yttrium. 



By permission of Mrs. Kunz, wife of the well known 

 New York mineralogist, I will show you perhaps the most 

 remarkable of all phosphorescing diamonds. This prodigy 

 diamond will phosphoresce in the dark for some minutes 

 after being exposed to a small pocket electric light, and if 

 rubbed on a piece of cloth a long streak of phosphorescence 

 appears. 



Triho-luminesccnce. 



.\ few minerals give out light when rubbed, and Mrs. 

 Kunz's diamond is equally striking in this respect. In the 

 year 16(13, 'he Hon. Robert Boyle read a paper before the 

 Royal Society, in which he described several experiments 

 made with a diamond which markedly showed tribo- 

 luminescence. As specimens of tribo-luminescent bodies, I 

 show you sphalerite (sulphide of zincl and an artificial 

 sphalerite, which is even more responsive to friction than 

 the native sulphide.' 



Combustion of the Diunwnil. 

 When heated in air or oxygen to a temperature vary- 

 ing from 760° C. to 875° C, according to its hardness, the 

 diamond burns with production of carbonic acid. It leaves 

 an extremely light ash, sometimes retaining the shape of 

 the crystal, consisting of iron, lime, magnesia, silica, and 

 titanium. In boart and carbonado the amount of ash 

 sometimes rises to 4 per cent., but in clear crystallised 

 diamonds it is seldom higher than 005 per cent. By far 

 the largest constituent of the ash is iron. 



.-Iction of liadinm on Diamond. 



The (3 rays from radium having like properties to the 

 stream of negative electrons in a radiant matter tube, it 

 was of interest to ascertain if they would exert a like 

 difference on diamond. The diamond glows under the in- 

 fluence of the radiations, and crushed diamond cemented 

 to a piece of card or metal makes an excellent screen in a 

 spinthariscope — almost as good as zinc sulphide. .Some 

 fine colourless crystals of diamond were embedded in 

 radium bromide and kept undisturbed for more than twelve 

 months. .\t the end of that time they were examined. 

 The radium had caused them to assume a beautiful blue 

 colour, and their value as " fancy stones " had been 

 materially increased. Here are a couple of diamonds 

 originally of the same purity of water. One has been 

 coloured by radium, the other is in its natural state. The 

 colour of the radium-tinted stone is very pronounced. The 

 lantern slide shows the darkening thus produced. A and B 

 are diamonds after twelve months' burial in radium 

 bromide ; diamond C is of the original colour. 



This blue colour is persistent, and penetrates below the 

 surface. It is unaffected by long-continued heating in 

 strong nitric acid and potassium chlorate, and is not dis- 

 charged by heating to redness. 



To find out if this prolonged contact with radium had 

 communicated to the diamond any radio-active properties, 

 six diamonds were put on a photographic plate, and kept 

 in the dark for a few hours. I will project the image of 

 the result after development. The three on the upper row 

 are the diamonds which have had a prolonged sojourn 

 with radium, the three below are similar diamonds picked 

 out for comparison, which have not been near radium. 

 See how strangely the three upper ones have acted. Notice 

 also that by mere contiguity to the others the lower 

 diamonds also shine with an induced, factitious radio- 

 activity. T throw on the screen a magnified image of one 

 of the blue crystals, and you see in how regular and 

 geometrical a pattern the radio-active emanations radiate 

 from the crystal. This observation has only been made 



' Artificial tribo-luminescent sphalerite : — 



Zinc carbonate too parts 



Flower ''f sulphur ... ... ... 30 ,, 



Manganese sulphate J per cent. 



Mix with distilled water and dry at a gentle heat. Put in luted crucible 

 and keep at a bright red heat for from two to three hours. 



