of a hydrogen atom which has lost its electron with that of one which has not, is a matter of very great importance; unfortunately it is extremely difficult to do it in a way which is free from ambiguity. On the view just given, the spectrum should be quite different; indeed we should hardly expect the atom when deprived of its electron to be able to give out any lines in the visible part of the spectrum. I have recently been able to show that when these positively charged atoms impinge on other atoms, they give rise to Röntgen rays; it will be interesting to compare the quality of these rays with those given out by the impact of cathode rays moving either with the same velocity or with the same energy.
The Structure of the Atom
We have seen that each atom contains a definite number of electrons, the number ranging from one for the hydrogen atom to over a hundred for the atom of thorium. The problem of deducing by mathematical consideration the way in which a number of electrons would arrange themselves when in stable equilibrium is one of fundamental importance. In our theoretical investigations of the structure of the atom it is well to keep constantly in our minds the question of the validity of applying to the problem of the individual atom principles which have been established by the study of the properties of collections of vast quantities of atoms. In the atom we have to deal with the electron and the corresponding charge of positive electricity; these are the units of which all electrical charges are built up. The laws of electric and magnetic action which we use in our theoretical investigations are based on the results of experiments, made not with a single unit of electricity, but with collections of millions of such units;