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.
To take a familiar instance, coal-gas is mainly "methane," one of the many combinations of carbon and hydrogen, and when it is burnt in air the hydrogen and oxygen combine, liberating heat, which causes the carbon liberated at the same time to glow. As each methane molecule breaks up the carbon atoms are thrown out, forming solid particles of carbon, and it is they really which give the light. It is therefore the combustible gas heating the solid particles of carbon which forms the luminous part of the gas flame. The non-luminous part of the flame, near the burner (I am now speaking of the old-fashioned burner), is the burning gas before the carbon particles have had time to heat up.
And the old gas flame, as we know, is now being rapidly displaced by the incandescent mantle, the reason being simply that Von Welsbach discovered how certain rare minerals gave a more brilliant light when heated than particles of carbon do. In other words, it is easier to accelerate the motion of the corpuscles in ceria, thoria and the other ingredients of the mantle, than it is those of carbon. Consequently, they sooner reach that degree of agitation which will send forth electro-magnetic waves of the high frequency necessary to produce the sensation of light.
For this reason the mantle heated by gas gives as bright a light as the carbon particles in the electric arc, although the latter are subjected to a much more intense heat.
But the arc can be, and often is, used as a source of heat, apart altogether from the light which it gives. In Sweden, for example, where coal is rare, but water-power plentiful, the power of the waterfalls is made to smelt iron. Hence the waterfalls are sometimes termed the "white coal" of that country. Needless to say, it is the ubiquitous electricity which performs the change from the force of falling water into heat.