GOLD AND DIAMONDS 687 GOLD 687 DIAMONDS 696 NEW METALS 714 INDIA-RUBBER AND GUTTA-PERCHA 724 INDIA-RUBBER 724 GUTTA-PERCHA 728 ANÆSTHETICS 731 EXPLOSIVES 740 MINERAL COMBUSTIBLES 751 COAL 751 PETROLEUM 757 PARAFFIN 761 COAL-GAS 764 COAL-TAR COLOURS 781 THE GREATEST DISCOVERY OF THE AGE 801 NOTES 811 INDEX 813 LIST OF ILLUSTRATIONS. FIG. PAGE Heading—Rain, Steam, and Speed (after Turner) 1 1. Portrait of James Watt 3 2. Newcomen’s Steam Engine 4 3. Watt’s Double-action Steam Engine 5 4. Governor and Throttle-Valve 6 4_a_. Watt’s Parallel Motion 8 5. Slide Valve 9 6. Section of Gifford’s Injector 11 7. Bourdon’s Pressure Gauge 12 8. Steam Generator 13 9. Section of Locomotive 15 10. Stephenson’s Link Motion 17 10_a_. G. N. R. Express Passenger Locomotive 19 10_b_. Joy’s Valve Gear 20 11. Locomotive after Explosion 22 12. Hancock’s Steam Omnibus 22 13. Nasmyth’s Steam Hammer 27 14. Merryweather’s Steam Fire-Engine 28 15. A Foundry 29 16. Aerolite in the British Museum 31