the positive going one way, and all the negative the other way; and each kind of matter possesses an intrinsic or characteristic ionic velocity, when urged by a given field through a given solution. The charges may be likened to horses or other propelling agency, and the atom to the vehicle or heavy body which is dragged along. The speed of travel through liquids is very slow, but through gases is considerably quicker, partly because there is less resistance, and partly because it is easier to maintain a steep gradient of potential in a medium where the ions are not too numerous.
The act of production of such ions is styled 'ionization,' and the process has been employed to explain very many facts in both Physics and Chemistry.
As an example, Röntgen rays passing through air ionize it and so render it conducting for a time: wherefore they are able readily to discharge electrified bodies, in this secondary way.
It may be convenient here to emphasize the dimensions of an electron as above specified, for the arguments in favour of that size are very strong though not absolutely conclusive; we are sure that their mass is of the order one-thousandth of the atomic mass of hydrogen, and we are sure that if they are purely and solely electrical their size must be one hundred-thousandth of the linear dimensions of an atom; a size with which their penetrating power and other behaviour is quite consistent. Assuming this estimate to be true, it is noteworthy how very small these electrical particles are, compared with the atom of matter to which they are attached. If an electron is represented by a sphere
