Such are the ever-shifting currents of the ocean of air. CHAPTER IX ATMOSPHERIC ELECTRICITY Every schoolboy has read how Benjamin Franklin, by means of his famous kite experiment, demonstrated the electrical nature of lightning, and how the same versatile genius invented the lightning rod. It is not proposed to repeat familiar history here. Neither shall we discuss the dubious statements frequently put forth that lightning rods were known before Franklin’s time, nor consider how much credit is due the many philosophers who, at earlier periods, suspected lightning to be a manifestation of electricity. The facts and ideas concerning atmospheric electricity that we have to present in this chapter were, for the most part, quite unknown to Franklin and to many generations of _savants_ after him, and some of them are just now finding their way into the textbooks. Science still recognizes the existence of two kinds of electricity--positive and negative--which, by combining, neutralize each other’s effects. According to current ideas, however, the more active agent in electrical phenomena is negative electricity, which is believed to consist of (or to provide electrical charges for) exceedingly minute particles called _electrons_. Only a few years ago the smallest thing that science had to deal with was the atom, and the lightest of atoms is that of hydrogen. The discovery of electrons marks a new step toward the infinitely little. The mass of the electron--or, in more popular and less exact terms, its weight--is about 1/1800 that of a hydrogen atom. As to its size: Imagine a billiard ball magnified to the size of the earth. Its constituent atoms would be the actual size of billiard balls, but the electrons of which each atom is composed would still be too small to be seen with the naked eye. Now imagine each of these billiard-ball atoms further magnified to the size of a large church. The electrons would then be about as big as one of the periods on this page. When we say that a body has an electrical charge we mean that it has an _excess_ of positive or negative electricity. An ordinary molecule of an atmospheric gas contains (or perhaps actually consists of) equal amounts of the two kinds of electricity, and is therefore not charged. There are, however, various ways in which an electron may be detached from such a molecule, leaving it positively charged; and again it may receive an extra electron, and thus acquire a negative charge. Under the former circumstances it becomes a _positive ion_, and under the latter a _negative ion_. Ions play a very important role as carriers of electricity, because they are impelled to move toward oppositely charged bodies or particles and combine with them. A gas containing ions is said to be _ionized_; and it is the ionization of the atmosphere that makes it a conductor of electricity.