Some of the causes which produce atmospheric electricity are stated under ELECTRICITY, ATMOSPHERIC. It will there be seen that the atmosphere is nearly always in a different electrical condition to the earth, the one being usually charged with positive electricity and the surface of the earth with negative. It was shown also that the electrical intensity increases with the height, that in stormy weather, in rain, hail, or snow, the electricity of the air is much more intense than at other times, and is liable to sudden changes from + to — and from — to+ . Now the effect of heat, evaporation, and other causes, is to exalt the electrical condition of the air, while a surface of cloud acts strongly by induction upon another opposed surface of cloud, or upon the opposed surface of the earth beneath, and the two opposed surfaces mutually reinforce each other. The cloud and the earth may thus be regarded as the terminal planes of a highly charged system of dielectric air, the tension of which goes on increasing until any further increase causes it to break down, when the opposite electricities rush together with violence, producing what is called a disruptive discharge, or in other words a flash of lightning, accompanied by thunder. During a thunder-storm the air between these two terminal planes may be charged and discharged a number of times until equilibrium is attained, or until the thiinder-clouds are borne away by the wind to another locality.
There aro various kinds of lightning, presenting different phenomena, and recognised under different names. In the first place there are those narrow well-defined ribbons or lines of light, moving iu a zigzag course, of a dazzling whito colour, but sometimes of a violet or purple hue ; these form, probably, the only kind of lightning that strikes ter restial objects. It proceeds from a single point, and frequently divides into two or more distinct streams. It is this bifurcation, or trifurca. tion, that has obtained for it the name of forked lightning. When form ing a long rippling line of light, sailors call it chain lightning. Some admirable illustrations of this, the first kind of lightning, are given under BREATII Flonur:s. When the lightning appears to be spread over an extensive surface, and varies in colour, being often red, but sometimes blue and violet, often confined to the edges of clouds, but when apparently issuing from the interior, the clouds are said to open, we have the variety known as sheet lightning. Lightning of this kind when unaccompanied by thunder is known as summer lightning, and is the reflection in the upper regions of the atmosphere of common lightning, having its origin in a storm, the direct view of which is obstructed by the rotundity of the earth. A third kind of lightning is the globular, which appears like a luminous ball or globe of fire ; it moves through the air at a comparatively slow rate, while lightnings of the first and second class exist but for a moment. Lightning of this kind sometimes obtains the name of fire-balls : it rolls along the surface with a hissing noise, and often terminates in a disruptive discharge.
This kind probably arises from a kind of glow or brush discharge in certain points of an excited system of air, preparatory to the more general and rapid union of the electrical forces. A milder form of this variety of lightning is known as St. Elmo's or St. Helmo's fire to the French and Spaniards, the fires of St. Peter and St. Nicholas to the Italians, while the Portuguese call it Corpos San los, which our English sailors seem to have corrupted into Comazants. It appears as tufts of light upon the points of ships' masts, the tips of spears, bayonets ; on the alpenstocks of Alpine travellers, on the tips of the fingers when the outspread hand is raised, and on any other pointed bodies, during a peculiarly excited state of the atmosphere, when the particles of air near the bodies in question discharge themselves upou the particles im mediately above them, the electrical condition of which is less intense, the effect being a luminous brush of light, accompanied by a slight roaring noise. This brush discharge is, in the language of that excellent electrician Sir W. Snow Harris, " an intermitting series of electrical sparks between metal and air, but in such rapid succession as to con vey the idea of a continuous stream." In some cases the appearance of the brush is changed into that of.a star, as when proceeding from negative or resinous electricity, when the discharge is towards the pointed con ductor, and not from it as in the case of the brush discharge. A kind of brush discharge usually precedes the-discharge of a Leyden jar, or of a thunder-cloud upon a ship or building, marking out, as it were, the line of least resistance for the principal discharge. [BREATH FIGURES.] When lightning strikes the earth the resulting effects are of the most varied character. it is a popular remark that the lightning-stroke is succeeded by a suffocating odour, which has been compared to that of burning sulphur. Scientific chemists have compared it to the odour of phosphorus, or to that of nitrous acid; but it is more than probable that this odour is due to the presence of OZONE, which is probably generated by the action of lightning on the oxygen of the air. The passage of lightning through the air causes a combination of its con stituent oxygen and nitrogen in the proportions to form citric acid It was shown by Cavendish that by passing artificial electrical dis charges through a confined portion of air this acid was formed. Liebig has also shown that in seventeen specimens of rain-water, collected during or immediately after thunder-storms, all contained nitric acid in greater or less quantities in combination with lime or with ammonia, while out of sixty specimens collected during ordinary rains, these sub stances were not found iu fifty-eight of them. The formation of ammonia (NH,) during thunder-storms shows that lightning decom 1 oses a portion of water. The presence of ammoniacal salts in storm rains is of great importance to vegetation, while the nitric acid of the storm is probably one of the sources of the natural nitres of the earth.