Electricity is evolved by bodies passing from a liquid to a solid state, also by the production and condensation of vapor, which is consequently a great source of atmospheric electricity.

The atmosphere, when clear, is almost always positively electric; its electricity is stronger in winter than in summer, during the day than in the night. The intensity increases for two or three hours from the time of sunrise, then decreases towards the middle of the day, and again augments as the sun declines, till about the time of sunset, after which it diminishes, and continues feeble during the night. Atmospheric electricity arises from an evolution of the electric fluid during the evaporation that is so abundant at the surface of the earth; and clouds probably owe their existence, or at least their form, to it, for they consist of hollow vesicles of vapor coated with electricity; as the electricity is either entirely positive or negative, the vesicles repel each other, which prevents them from uniting and falling down in rain. The friction of the surfaces of two strata of air moving in different directions, probably developes electricity; and if the strata be of different temperatures, a portion of the vapor they always contain will be deposited; the electricity evolved will be taken up by the vapor, and will cause it to assume the vesicular state constituting a cloud. A vast deal of electricity may be accumulated in this manner, which may either be positive or negative, and should two clouds charged with opposite kinds approach within a certain distance, the thickness of the coating of electricity will increase on the two sides of the clouds that are nearest to one another; and when the accumulation becomes so great as to overcome the coercive pressure of the atmos

phere, a discharge takes place, which occasions a flash of lightning. The actual quantity of electricity in any one part of a cloud is extremely small; the intensity of the flash arises from the very great extent of surface occupied by the electricity, so that the clouds may be compared to enormous Leyden jars thinly coated with the electric fluid, which only acquires its intensity by its instantaneous condensation.

An interchange frequently takes place between the clouds and the earth, but on account of the extreme rapidity of lightning it is difficult to ascertain whether it goes from the clouds to the earth, or shoots upwards from the earth to the clouds, though there can be no doubt that it does both. M. Halvig measured the velocity of lightning by means of the camera lucida, and estimates that it is probaably eight or ten miles in a second, or about forty times greater than that of sound; and M. Gay-Lussac has ascertained that a flash of lightning sometimes darts more than three miles at once in a straight line.

A person may be killed by lightning, although the explosion takes place at the distance of twenty miles, by what is called the back stroke. Suppose that the two extremities of a cloud highly charged with electricity hang down towards the earth, they will repel the electricity from the earth's surface, if it be of the same kind with their own, and will attract the other kind; and if a discharge should suddenly take place at one end of the cloud, the equilibrium will instantly be restored by a flash at that point of the earth which is under the other.

The pure air, at all times negatively electric, becomes intensely so on the approach of rain, snow, wind, hail, or sleet, but it afterwards varies on opposite sides, and the

transitions are very rapid on the approach of a thunderstorm. An insulated conductor then gives out such quantities of sparks that it is dangerous to approach it, as was fatally experienced by Professor Richman, at Petersburg, who was struck dead by a globe of fire from the extremity of a conductor, while making experiments on atmospheric electricity. There is no instance on record of an electric cloud being dispelled by a conducting rod silently withdrawing the electric fluid; yet it may mitigate the stroke, or render it harmless if it should come. Sir John Leslie observes, that the efficacy of conductors depends upon the rapidity with which they transmit the electric energy; and as copper is found to transmit the fluid twenty times faster than iron, and as iron conducts it 400000000 times more rapidly than water, which conveys it several thousand times faster than dry stone, copper conductors afford the best protection, especially if they expose a broad surface, since the electric fluid is conveyed chiefly along the exterior of bodies. The object of a conductor being to carry off the electricity in case of a stroke, and not to invite an enemy, it ought to project very little, if at all, above the building.

The aurora borealis is decidedly an electrical phenomenon, which takes place in the highest regions of the atmosphere, since it is visible at the same time from places very far distant from each other. It is somehow connected with the magnetic poles of the earth, but it has never been seen so far north as the pole of the earth's rotation, nor does it extend to low latitudes. It generally appears in the form of a luminous arch, stretching more or less from east to west, but never from north to south; across the arch the coruscations are rapid, vivid,

and of various colors. A similar phenomenon occurs in the high latitudes of the southern hemisphere. Mr. Faraday conjectures that the electric equilibrium of the earth is restored by means of the aurora conveying the electricity from the poles to the equator.

SECTION XXIX.

Galvanism is a peculiar kind of electricity, elicited by the force of chemical action, instead of friction. It is connected with one of the most brilliant periods of British science, from the splendid discoveries to which it led Sir Humphrey Davy; but it has acquired additional interest since it has proved, by the reciprocal action of galvanic and magnetic currents, that magnetism has no existence as a distinct or separate principle, but is only an effect of electricity therefore, galvanism, as immediately connected with the theory of the earth and planets, forms a part of the physical account of their nature.

The disturbance of electric equilibrium, and a development of electricity, invariably accompanies the chemical action of a fluid on metallic substances, and is most plentiful when that action occasions oxidation. Metals vary in the quantity of electricity afforded by their combination with oxygen; but the greatest abundance is developed by the oxidation of zinc by weak sulphuric acid; and in conformity with the law, that one kind of electricity cannot be evolved without an equal quantity of the other being brought into activity, it is found that the acid is positively, and the zinc negatively electric. It has not yet been ascertained why equilibrium is not restored by the contact

of these two substances, which are both conductors, and in opposite electrical states; however, the electrical and chemical changes are so connected, that unless the equilibrium be restored, the action of the acid will go on languidly, or stop as soon as a certain quantity of electricity is accumulated in the acid. The equilibrium, however, will be restored, and the action of the acid will be continuous, if a plate of copper be placed in contact with the zinc, both being partly immersed in the fluid; for the copper, not being acted upon by the acid, will serve as a conductor to convey the positive electricity from the acid to the zinc, and will at every instant restore the equilibrium, and then the oxidation of the zinc will go on rapidly. Thus three substances are concerned in forming a galvanic circuit, but it is indispensable that one of them be a fluid. The electricity so obtained will be very feeble, but it may be augmented by increasing the number of plates. In the common galvanic battery, the electricity which the fluid has acquired from the first plate of zinc exposed to its action, is taken up by the copper plate belonging to the second pair, and transferred to the second zinc plate with which it is connected. This second plate of zinc having thus acquired a larger portion of electricity than its natural share, communicates a larger quantity of electricity to the fluid in the second cell. This increased quantity is again transferred to the next pair of plates; and thus every succeeding alternation is productive of a further increase in the quantity of the electricity developed. - This action, however, would stop unless a vent were given to the accumulated electricity, by establishing a communication between the positive and negative poles of the battery, by means of wires attached to the extreme plate at