not until quite the close of the nineteenth century, when attention was turned to the study of electrified atoms instead of unelectrified ones, that our acquaintance with the atom became at all intimate. The advance made through the electrification of the atom has been most remarkable; it is due to the fact that an unelectrified atom is so elusive that unless more than a million million are present we have no means sufficiently sensitive to detect them, or, to put it in another way, unless we had a better test for a man than we have for an unelectrified molecule, we should be unable to find out that the earth was inhabited. The electrified atom or molecule, on the other hand, is much more assertive, so much so that it has been found possible in some cases to detect the presence of a single electrified atom; a billion unelectrified atoms may escape our observation, whereas a dozen or so electrified ones are detected without difficulty.
One reason why electrified atoms and molecules are so much easier to study is that we can subject them to forces far more intense than any we can apply to unelectrified ones; we can exert much more control over them, and force them into situations where their habits may be observed. For example, if a mixture of different kinds of electrified atoms is moving along in one stream then when electric and magnetic forces are applied to the stream simultaneously, the different kinds of atoms are sorted out, and the original stream is divided up into a number of smaller streams separated from each other. The particles in any one of the smaller streams are all, of the same kind.
Thus, if the original stream contained a mixture of hydrogen and oxygen atoms, it would, by the action of the electric and magnetic forces, be split up into two separate
