A powerful galvanic battery is capable of producing light of intense brilliancy. For this purpose, instead of taking sparks between the two metallic poles, each of these is connected with a piece of carbon (which is nearly as good a conductor as the metal), and the sparks are taken between these two pieces of carbon, usually set so that the one connected with the negative pole is virtually above the one connected with the positive pole, and at a distance of a tenth of an inch from each other or more, according to the strength of the battery. Across this gap between the carbons an arc of light is seen, which in reality results from a series of electric sparks following each other in rapid succession. This arc, called the voltaic arc, is brilliant, but it is not from this arc that the chief part of the light comes. The ends of the carbon become intensely bright, being raised to a white heat. Both the positive and negative carbons are fiercely heated, but the positive is heated most. As (ordinarily) both carbons are thus heated in the open air, combustion necessarily takes place, though it is to be noticed that the lustre of the carbons is not due to combustion, and would remain undiminished if combustion were prevented. The carbons are thus gradually consumed, the positive nearly twice as fast as the negative. If they are left untouched, this process of combustion soon increases the distance between them till it exceeds that which the electricity can pass over. Then the light disappears, the current ceasing to flow. But by bringing the carbon points near to each other (they must, indeed, be made to touch for an instant), the current is made to flow again, and the light is restored. The following remarks by M.H. Fontaine (translated by Dr. Higgs) may help to explain the nature of the voltaic arc:--'In truth, the voltaic arc is a portion of the electric circuit possessing the properties of all other parts of the same circuit. The molecules swept away from point to point (that is, from one carbon end to the other) 'constitute between these points a mobile chain, more or less conductive, and more or less heated, according to the intensity of the current and the nature and separation of the electrodes' (that is, the quality and distance apart of the carbon or other substances between which the arc is formed). 'These things happen exactly as if the electrodes were united by a metallic wire or carbon rod of small section' (so as to make the resistance to the current great), 'which is but saying that the light produced by the voltaic arc and that obtained by incandescence arise from the same cause--that is, the heating of a resisting substance interposed in the circuit.'