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 they represent in fact the average effect of millions of individuals. When, however, we come to the atom, we have to deal with the effects produced by the individual electron or positive charge, and not with the average effect produced by countless numbers of such charges. Now it may be or it may not be that the average effect is identical with that produced by each individual, and it may be or it may not be that a knowledge of the average is sufficient to solve the problem of the individual. The statistician is content to know that the average height of male adults is, say, 5 feet 6 inches, and their waist measurement 3 feet, but it is evident that such knowledge would be a very unsatisfactory equipment for one's tailor. Now the laws of electricity and magnetism as stated in our text-books are statistical laws, and when we come to apply them to the atom we are somewhat in the position of a tailor attempting to fit an individual with nothing but a knowledge of the average dimensions of the whole population to go upon. We must, therefore, proceed in a somewhat tentative fashion, and try if our statistical knowledge, which is all we have at present, will ensure a fit for the atom; we need not, however, be very much surprised if the fit is not perfect, and we must, by the means which fortunately are now at our disposal for the study of the properties of the electron and the positive charge, endeavour to supplement our statistical knowledge by the knowledge of the effect produced by each individual. I think that the most pressing need at this stage of the Atomic Theory is the exploration by experiment of the distribution of electrons in the atom; when we know this distribution we may be able to see how we must modify the accepted laws of electrical action to make them applicable to these small charges.

We may, I think, get a useful lesson by considering