594 



NA TURE 



[OcTOliEK 12, 1905 



tinctorial powers. One batch of iron might contain an 

 impurity colouring the stones blue, another lot would tend 

 towards the formation of pink stones, another of green, 

 and so on. Cobalt, nickel, chromium, and manganese, all 

 metals present in the blue ground, would produce these 

 colours. 



kn hypothesis, however, is of little value if it only 

 elucidates half a problem. Let us see how far we can 

 follow out the ferric hypothesis to explain the volcanic 

 pipes. In the first place we must remember these so-called 

 volcanic vents are admittedly not filled with the eruptive 

 rocks, scoriaceous fragments, &c., constituting the ordinary 

 contents of volcanic ducts. 



■Selections of thin sections of some of these rocks and 

 minerals, mounted as microscopic objects and viewed by 

 polarised light, are not only of interest to the geologist, 

 but are objects of great beauty. 



The appearance of shale and fragments of other rocks 

 testifies that the melange has suffered no great heat in its 

 present condition, and that it has been erupted from great 

 depths by the agency of water vapour or some similar 

 gas. How is this to be explained? 



You will recollect I start with the reasonable supposi- 

 tion that at a sufficient depth ' there were masses of 

 molten iron at great pressure and high temperature, hold- 

 ing carbon in solution, ready to crystallise out on cooling. 

 Far back in time the cooling from above caused cracks in 

 superjacent strata through which water' found its way. 

 On reaching the incandescent iron, the water would be 

 converted into gas, and this gas would rapidly disintegrate 

 and erode the channels through which it passed, grooving 

 a passage more and more vertical in the necessity to find 

 the quickest vent to the surface. But steam in the 

 presence of molten or even red-hot iron liberates large 

 volumes of hydrogen gas. together with less quantities of 

 hydrocarbons ° of all kinds — liquid, gaseous, and solid. 

 Erosion commenced bv steam would be continued by the 

 other gases ; it would be easy for pipes, large as any 

 found in South Africa, to be scored out in this manner. 



.Sir .\ndrew Noble has shown that when the screw 

 stopper of his steel cylinders in which gunpowder explodes 

 under pressure is not absolutely perfect, gas escapes with 

 a rush so overpowering as to score a wide channel in the 

 metal. Some, of these stoppers and vents are on the table. 

 To illustrate my argument .Sir Andrew Noble has been 

 kind enough to try a special experiment. Through a 

 cylinder of granite is drilled a hole 0-2 inch diameter, the 

 size of a small vent. This is made the stopper of 

 an explosion chamber, in which a quantity of cordite 

 is fired, the gases escaping through the granite 

 vent. The pressure is about 1500 atmospheres, and 

 the whole time of escape is less than half a second. 

 Notice the erosion produced by the escaping gases 

 and by the heat of friction ; these forces have scored 

 out a channel more than half an inch diameter and melted 

 the granite along their course. If steel and granite are 

 thus vulnerable at comparatively moderate gaseous 

 pressure, it is easy to imagine the destructive upburst of 

 hydrogen and water-gas grooving for itself a channel in 

 the diabase and quartzite, tearing fragments from resist- 

 ing rocks, covering the country with debris, and finally, 

 at the subsidence of the great rush, filling the self-made 

 pipe with a water-borne magma in which rocks, minerals, 

 iron oxide, shale, petroleum, and diamonds are violently 

 churned in a veritable witch's cauldron ! As the heat 

 abated the water vapour would gradually give place to 

 hot water, which forced through the magma would change 

 some of the mineral fragments into the existing forms of 

 to-day. 



Each outbreali would form a dome-shaped hill ; the 

 eroding agency of water and ice would plane these emin- 

 ences until all traces of the original pipes were lost. 



.Actions such as I have described need not have taken 

 place simultaneously. ,\s there must have been many 



1 A pressure of fifteen tons on t^e square inch would exisl not many miles 

 beneath the surface of the earth. 



- There are abundant .^igns that a considerable poition of this part of 

 .'Africa was once underwater, and a fr*sh-water shell has been found in 

 apparently undisturbed blue ground at Kimb;rley. 



■* I he water sunk in we Is close to the Kimberley mine is sometimes im- 

 pregnated with paraffin, and Sir H. Ro^coc extracted a solid hydrocarbon 

 from the '' blue ground." 



molten masses of iron with variable contents of carbon, 

 different kinds of colouring matter, solidifying w-ith vary- 

 intf degrees of rapidity, and coining in contact with water 

 at" intervals throughout long periods of geological time- 

 so must there have been many outbursts and upheavaU, 

 giving rise to pipes containing diamonds. And the-( 

 diamonds, bv sparseness of" distribution, crystalline 

 character, difference of tint, purity of colour, varym- 

 hardness, brittleness,. and state of tension, have the stor\ 

 of their origin impressed upon them, engraved by natural 

 forces — a story which future generations of scientific mm 

 may be able to interpret with greater precision than i- 

 possible to-day. 



■ Gcnvih of the Diamond. 

 Speculations as to the probable origin of the diamond 

 have been greatly forwarded by patient research, ami 

 particularly by imjjroved means of obtaining high temper.i- 

 tures, an advance we owe principally to the researches "\ 

 Prof.' Moissan. 



Until recent years carbon was considered absolutely nmi 

 volatile and infusible ; but the enormous temperatures ai 

 the disposal of experimentalists — by the introduction of 

 electricity — show that, instead of breaking rules, carbon 

 obeys the same laws that govern other bodies. It 

 volatilises at the ordinary pressure at a temperature of 

 about 3600° C, and passes from the solid to the gaseous 

 state without liquefying. It has been found that other 

 bodies, such as arsenic, which volatilise without liquefying 

 at the ordinary pressure, will easily liquefy if pressure is 

 added to temperature. It naturally follows that if along 

 with the requisite temperature sufficient pressure is applied, 

 liquefaction of carbon will take place, when on cooling it 

 will crystallise. But carbon at high temperatures is a 

 inost energetic chemical agent, and if it can get hold of 

 o.xvgen from the atmosphere or any compound containing 

 ' it, it will oxidise and fly off in the form of carbonic acid. 

 Heat and pressure, therefore, are of no avail unless the 

 carbon cjin be kept inert. 



It has long been known that iron when melted dissolves 

 carbon, and on cooling liberates it in the form of graphite. 

 Moissan discovered that several other inetals, especially 

 silver, have similar properties ; but iron is the best solvent 

 for carbon. The quantity of carbon entering into solution 

 increases with the temperature. 



For the manufacture of — I am afraid I must say an 

 infinitesimal — diamond, the first necessity is to select pure 

 iron — free from sulphur, silicon, phosphorus, &c. — and to 

 pack it in a carbon crucible with pure charcoal from sugar. 

 The crucible is then put into the body of the electric 

 furnace, and a powerful arc formed close above it between 

 carbon poles, utilising a current of 700 amperes at 40 volts 

 pressure. The iron rapidly melts and saturates itself with 

 carbon. .After a few minutes' heating to a temperature 

 above 4000° C. — a temperature at which the iron melts like 

 wax and volatilises in clouds — the current is stopped, and 

 the dazzling fiery crucible is plunged beneath the surface 

 of cold water, where it is held until it sinks below a red 

 heat. As is well known, iron increases in volume at the 

 moment of passing from the liquid to the solid state. 

 The sudden cooling solidifies the outer layer of iron and 

 holds the inner molten mass in a tight grip. The ex- 

 pansion of the inner liquid on solidifying produces an 

 enormous pressure, and under the stress of this pressure 

 the dissolved carbon separates out in transparent forrns — 

 minutely microscopic, it is true — all the same veritable 

 diamonds, with crystalline form and appearance, colour, 

 hardness, and action on light the same as the natural gem. 

 Now commences the tedious part of the process. The 

 metallic ingot is attacked with hot nitro-hydrochloric acid 

 until no more iron is dissolved. The bulky residue con- 

 sists chiefly of graphite, together with translucent chestnut- 

 coloured flakes of carbon, black opaque carbon of a density 

 of from 3-0 to 3.5, and hard as diamonds — black diamonds 

 or carbonado, in fact — and a small portion of transparent 

 colourless diamonds showing crystalline structure. Besides 

 these, there may be carbide of silicon and corundum, 

 arising from iinpurities in the materials employed. 



The residue is first heated for some hours with strong 

 sulphuric acid at- the boiling point, with the cautious 

 addition of powdered nitre. It is then well washed, and 

 for two days allowed to soak in strong hydrofluoric acid 



NO. 1876, VOL. JZ] 



