A conjecture has recently been started that electricity may be generated by the mere impact of minute particles of water vapor against minute particles of air.[13] If this conjecture could be established as a fact, it would be amply sufficient to account for all the electricity of the atmosphere. From the very nature of a gas, the molecules of which it is composed are forever flying about with incredible velocity; and therefore the particles of water vapor and the particles of air, which exist together in the atmosphere, must be incessantly coming into collision. Hence, however small may be the charge of electricity developed at each individual impact, the total amount generated over any considerable area, in a single day, must be very great indeed. It is evident, however, that this method of explaining the origin of atmospheric electricity can only be regarded as, at best, a probable hypothesis, until the assumption on which it rests is supported by the evidence of observation or experiment. =Length of a Flash of Lightning.=--It would seem, then, that we are not yet in a position to indicate with certainty the sources from which the electricity of the atmosphere is derived. But whatever these sources may be, there can be little doubt that the electricity of the atmosphere is intimately associated with the minute particles of water vapor of which the thundercloud is eventually built up. This consideration is of great importance when we come to consider the special properties of lightning, as compared with other forms of electricity. The most striking characteristic of lightning is the wonderful power it possesses of forcing its way through the resisting medium of the air. In this respect it incomparably surpasses all forms of electricity that have hitherto been produced by artificial means. The spark of an ordinary electric machine can leap across a space of three or four inches; the machine we have employed in our experiments to-day can give, under favorable circumstances, a spark of nine or ten inches; the longest electric spark ever yet produced artificially is probably the spark of Mr. Spottiswoode’s gigantic induction coil; and it does not exceed three feet six inches. But the length of a flash of lightning is not to be measured in inches, or in feet or in yards; it varies from one or two miles, for ordinary flashes, to eight or ten miles in exceptional cases.