388. =Light of Electricity.=--The light produced by electricity is not occasioned by any thing like combustion. It depends obviously upon the resistance which is offered to its passage. Thus when the electric fluid passes through air from the prime conductor to the knob of the Leyden jar it causes a flash of light, but when it arrives at the knob the flash ceases. What is the reason of the difference? In both cases it has the resistance of the air, for when it comes to the knob it passes over the _surface_ of the knob and rod; but in the latter case it is so diffused in its conduction over the metallic surface that it meets with much less resistance from the air. By experiments with the air-pump it is found that the denser the air is the more vivid is the spark; and if electricity be passed through a glass vessel from which the air has been mostly exhausted we have the streams of light seen in the aurora borealis, which are so strikingly in contrast with the vivid flashes of the lightning. In the experiment, § 381, in which the word light is made by the passing electricity, we have a striking illustration of the production of the spark by the resistance of the air. If the foil were one continuous surface the electricity would be diffused over it without giving any light. It is only where the electric fluid has to leap through the air from one portion of foil to another that the light is seen. 389. =Sound of Electricity.=--The report of electricity is a sort of crack or snap from the sudden condensation of the air by the rapid passage of the fluid. The rolling of thunder is occasioned by the reverberation of the first sound among the clouds. The nearer the flash is to us the more like a crack is its first sound as it comes to our ears. 390. =Mechanical Injuries from Electricity.=--When any great amount of electricity meets in its passage with any imperfect conductor it does much violence to it. Thus it rends wood, scatters water, breaks glass, etc. Various experiments have been tried illustrating the manner in which mechanical injuries result from electricity. Thus if it be made to pass through a card or several leaves closely pressed together, there is a burr on each side of such a character as to show that two forces moving in opposite directions have made their passage.