electrons equal in number to the electro-positive valency of the element. The electro-positive valency is the valency when the element is acting as the electro-positive constituent of a compound, and, as Abegg pointed out, is in many cases connected with the electro-negative valency by the rule that the sum of the two valencies is equal to eight. An atom with n mobile electrons in the outer ring, or more generally one with an outer, ring of electrons so constituted that when n of its electrons are fixed the others also lose their mobility, would in its relation to other atoms show the properties which the chemists describe by saying that the electro-positive valency of the atom is n.
I have alluded to several ways of investigating the structure of the atom; they one and all involve great labour, and any one who has used them must often have felt what a boon it would have been if we had an eye which would enable us to have a good look at an atom and have done with it. Now I cannot say that any such eye has been invented, but Mr. C. T. R. Wilson has made some approach to it by a beautiful method by which we can see, not indeed the individual atom itself, but still the path of such an atom, and in some cases what is going on in the atom. The method is based on the principle that when charged atoms or electrons are produced in air sufficiently supersaturated with water vapour, the water condenses on them and nowhere else. Thus each atom or electron is surrounded by a little drop of water, and the regions where they are produced are mapped out by threads of little drops of water resembling seed pearls; these can be photographed and studied at leisure. Now an electrified atom or electron travelling through a gas when it strikes against the atoms knocks