Sir Andrew 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 and a temperature so high as to score a wide channel in the metal. To illustrate my argument Sir Andrew Noble has been kind enough to try a special experiment. Through a cylinder of granite he drilled a hole 0·2 inch diameter, the size of a small vent. This was made the stopper of an explosion chamber, in which a quantity of cordite was fired, the gases escaping through the granite vent. The pressure was about 1500 atmospheres and the whole time of escape was less than half a second. The erosion produced by the escaping gases and by the heat of friction scored out a channel more than half an inch diameter and melted the granite along the 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 resisting 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 outbreak would form a dome-shaped hill; the eroding agency of water and ice would plane these eminences until all traces of the original pipes were lost. Actions such as I have described need not have taken place simultaneously. As there must have been many molten masses of iron with variable contents of carbon, different kinds of colouring matter, solidifying with varying degrees of rapidity, and coming in contact with water at intervals throughout long periods of geological time--so must there have been many outbursts and upheavals, giving rise to pipes containing diamonds. And these diamonds, by sparseness of distribution, crystalline character, difference of tint, purity of colour, varying hardness, brittleness, and state of tension, have the story of their origin impressed upon them, engraved by natural forces--a story which future generations of scientific men may be able to interpret with greater precision than is possible to-day. CHAPTER XI METEORIC DIAMONDS Sensational as is the story of the diamond industry in South Africa, quite another aspect fixes the attention of the chemist. The diamonds come out of the mines, but how did they get in? How were they formed? What is their origin? Gardner Williams, who knows more about diamonds than any man living, is little inclined to indulge in speculation. In his fascinating book he frankly says: