Generally speaking, clouds are, like the atmosphere, charged with positive electricity. Sometimes, however, there are negative clouds. You may frequently see, on the summits of mountains, clouds which seem to cling to the peaks for a while, as though drawn to them by some force of attraction, and then move away to follow the general direction of the winds. It often happens that in this case the clouds have lost their positive electricity in thus coming in contact with the mountains, and have derived from them in its place the negative electricity which, instead of holding them, has a tendency to drive them off. A mass of clouds lying between the negative earth and a mass of positive clouds above is almost neutral; the positive electricity accumulates towards its lower surface, and the first drops of rain will make it disappear. This mass will, from that moment, become like the surface of the soil--that is to say, it will become negative under the influence of the mass above it, endowed with a strong positive tendency. The cloud remains suspended in space until the moment when, under the influence of the ambient medium, it dissolves in rain. The causes of the instability of clouds are very numerous. My readers are aware that the atmosphere is being constantly agitated by vast currents which pass from the equator to the poles, and from which the different winds result. The clouds take part in this universal whirl of atmospheric waves. Transported from one point to another--often far beyond the region where they came into existence--subjected to every vicissitude of atmosphere, and blown about by contrary currents, they follow the gigantic movements which take the form sometimes of cyclones and tempests. Under the influence of warmth, and probably also by its transformation, these movements engender great masses of electricity, and presently, when the clouds have become saturated with it, the electricity breaks out, and there is a thunderstorm. The electric fluid, escaped from the cloud in which it has been imprisoned, flies to unite itself, either with the negative electricity stored in the surface of the earth, or else with the electricity in other neighbouring clouds. Almost always the cloud torn open by the electric discharge dissolves in rain or hail. Thus a storm is the outcome of violent movements produced by the force of electricity when this has reached its maximum of intensity. Thunderstorms are generally heralded by certain premonitory signs. The barometer goes down steadily. The air, calm and heavy, is pervaded by a bitter sulphurous odour. The heat is stifling. An abnormal silence reigns over the land. All this has a remarkable effect upon certain organisms, and produces nervous complaints, a buzzing in the ears, a sense of painful oppression, a sort of good-for-nothingness that we combat in vain.