[Footnote 3: We may speak of two electricities or two electric states without necessarily implying adherence either to the single or the double "fluid" theory. Whether electricity be of two kinds or no, the fact remains that there are two conditions, and all the features of this paper may be explained with equal facility by the supporters of either hypothesis.]
Should the cloud be near the earth, or should it be very highly charged, the tension of the two electricities may be so great as to overcome the resistance of the intervening air; and if this resistance should prove too weak, what happens? How does the discharge show itself? It takes place in the form of a lightning flash, and passing from the one surface to the other--or, maybe, simultaneously from both--produces neutrality more or less complete.
There has recently been a little discussion in these pages on the subject of lightning, some having stated that they discerned the discharge to take place upward--that is, from the earth toward the cloud. I will not venture so far as to say whether or not the direction of the discharge is discernible; possibly the flash may sometimes be long enough to enable one to tell; but I have never so seen it, and have always looked upon the eye as a deceitful member--very. "The lightning flash itself never lasts more than 1/100000 of a second." It is, however, just as likely that a discharge may travel upward as downward. What controls the discharge? Does the quality of the charge?--that is to say, is the positive or the negative more prone to break disruptively through the insulating medium? Investigations with Geissler's and other tubes containing highly rarefied gases have made it tolerably clear that there is a greater "tearing away" influence at the negative than at the positive pole, and if two equal balls, containing one a positive and the other a negative charge, be equally heated, the negative is more readily dissipated than the positive. But, so far as we at present know, this question enters into the discussion scarcely, if at all. Our knowledge seems rather to point to the substances upon which the charges are collected. The self-repellent nature of electricity compels it to manifest itself at the more prominent parts of the surface, the level being forsaken for the point. The tension of the charge, or its tendency to fly off, is proportionately increased. And if at a given moment the tension attains a certain intensity, the discharge follows, emanating from the surface which offers the greatest facilities for escape. The earth is generally flatter than the cloud, whence, in all probability, the discharge more frequently originates with the cloud.