And there is yet another possibility, though less alluring than what has just been described. The American writer mentioned a little while back got a better return from the mud of a creek than from the water itself. In all probability this is due to the action of organic matter carried down by streams, or in some other way introduced into the waters of the creek whence the mud was obtained. This organic matter would possibly have an effect as a precipitant upon the dissolved gold, causing it to be thrown out of solution and deposited in the mud. Thus the mud around our shores, and particularly in the creeks and estuaries, may be potential gold mines whence in time to come we may draw supplies of the precious metal. The cyanide or some similar process may be needed in order that we may extract the metal from its enclosing mud, but the time may not be so very far distant when dredging for gold may be a regular occupation at, for example, the mouths of the Thames and the Hudson. CHAPTER X INTENSE HEAT Many of the useful and interesting manufacturing processes of to-day are based upon the intense heat which science has taught the manufacturer how to produce. Tasks which our forefathers dreamed of, but were unable to accomplish, are easy to-day because of the facility with which great heat can be generated. The "burning fiery furnace" "seven times heated" is as nothing to some of the temperatures which are now obtained in the ordinary course of things. The greatest heat of all is that of the electric arc. Two conductors, generally rods of carbon, are placed with their ends touching, and the current is turned on so that it passes from one to the other. Then they are gradually drawn apart. As the gap widens the current experiences more and more difficulty in passing over this non-conducting gap, and great electrical energy has to be employed to keep it going. Now that wonderful law of the Conservation of Energy decrees that no energy can ever be lost. It can only be changed from one form into another. Therefore the energy expended upon the arc is not lost, but is converted into heat. It is that heat, acting upon the small particles of carbon which are torn off the ends of the rods, which gives us the arc light. As a matter of fact nearly all artificial light (and natural light too for that matter[1]) is due to heat. The heat sets the molecules in violent agitation, which, acting upon the corpuscles in the atoms, sets them in violent motion too, so that light is often the companion of heat. Some substances give light more readily than others, under the influence of heat, and we may reasonably believe that they are those whose corpuscular arrangements are such that they can be readily accelerated by the molecular action. [1] The glow-worm is an example of the few exceptions.