the way, has been used to measure the number of air molecules in the sky. Now when we know the mass and charge on an electron we can calculate the amount of hard Röntgen rays scattered by a single electron. Then if we measure the scattering due to the electrons in an atom, or in a million atoms, we shall be able to deduce the number of electrons in the atom. Measurements of the scattering of Röntgen rays were first made by Barkla, and from his results it follows that the number of electrons in an atom is roughly proportional to the atomic weight, and that the actual number is not very far from half the atomic weight; thus in the carbon atom there would be six electrons, in the oxygen atom eight, and so on, while in the lightest atom, hydrogen, there is probably only one. This is a most interesting result when we remember that there is room for 1,700 of these corpuscles in an atom of hydrogen, and that one of the spectra of hydrogen is of exceptional complexity.
Sir Ernest Rutherford by an entirely different method found that the quantity of positive electricity in an atom of atomic weight A is equal to the quantity of negative electricity in A/2 electrons. This also proves that the number of electrons in an atom is half the atomic weight.
The atomic weights of a great many elements are not divisible by two, so that the number of electrons in the atoms cannot be exactly equal to half the atomic weight. As the average difference between the atomic weights of successive elements is about 2, one-half the atomic weight of an element is not very far from its place in the list of elements arranged in order of the atomic weights; this place is called the atomic number of the element. Mr. van Broek has suggested that the number of electrons in an element is equal to its atomic number, and this
