In two words, our planet and its aerial envelope are two great reservoirs of electricity, between which take place continual exchanges which play a _rôle_ in the life of plants and animals complementary to that which is played by warmth and moisture. The aurora borealis, which sometimes illumines, with a brilliancy as of fairyland, the darkness of night in the Arctic and all the regions of the North, finds its explanation in the same phenomenon. It also is a striking of a balance, silent but visible, between two opposing tensions of the atmosphere and the earth; thus the apparition of the aurora borealis in Sweden or Norway is accompanied by electric currents moving through the earth to a distance sufficiently great to cause the magnetic needle to record the occurrence in the Paris Observatory. Indeed, the electricity which pervades the earth, silently and invisibly, is identical with that which moves in the heights of the enveloping atmosphere, and, whether it be positive or negative, its essential unity remains the same, these qualities serving only to indicate a point, more or less in common, between the different charges. The heights of the atmosphere are more powerfully electrified than the surface of the globe, and the degree of electricity increases in the atmosphere with the distance from the earth. Atmospheric electricity undergoes, like warmth, and like atmospheric pressure, a double fluctuation, yearly and daily, as well as accidental fluctuations more considerable than the regular ones. The maximum comes between six and seven in the morning in summer, and between ten and twelve in winter; the minimum comes between five and six in the afternoon in summer, and about three in the afternoon in winter. There is a second maximum at sunset, followed by a diminution during the night until sunrise. This fluctuation is connected with that of the hygrometric condition of the air. In the annual fluctuation the maximum comes in January, and the minimum in July; it is due to the great atmospherical circulation; the winter is the time when the equatorial currents are most active in our hemisphere, and when the aurora borealis is to be seen most often. On the other hand, the water of oceans and rivers is continually evaporating under the influence of solar heat, and rises into the atmosphere, where it remains in the form of an invisible gaseous vapour. Soon it becomes cold again, and, in the process of condensation, transparent gaseous molecules become transformed into minute drops, which accumulate into a cloud.