Since these enormously high X-ray frequencies must arise from electrons which fall into extraordinarily powerful fields of force, such as might be expected to exist in the inner regions of the atom close to the nucleus, Moseley’s discovery strongly suggests that the charge on this nucleus is produced in the case of each atom by adding some particular invariable charge to the nucleus of the atom next below it in Moseley’s table. This suggestion gains added weight when it is found that with one or two trifling exceptions, to be considered later, Moseley’s series of increasing X-ray frequencies is exactly the series of increasing atomic weights. It also receives powerful support from the following discovery. Mendeleéff’s periodic table shows that the progression of chemical properties among the elements coincides in general with the progression of atomic weights. Now it was pointed out ten years ago that whenever a radioactive substance loses a doubly charged -particle it moves two places to the left in the periodic table, while whenever it loses a singly charged -particle it moves one place to the right,[149] thus showing that the chemical character of a substance [Pg 203] depends upon the number of free positive charges in its nucleus. One of the most interesting and striking characteristics of Moseley’s table is that all the known elements between sodium (atomic number 11, atomic weight 23) and lead (atomic number 82, atomic weight, 207.2) have been fitted into it and there are left but three vacancies within this range. Below sodium there are just 10 known elements, and very recent study[150] of their spectra in the extreme ultra-violet has fixed the place of each in the Moseley progression, though in this region the progression of atomic weights and of chemical properties is also altogether definite and unambiguous. It seems highly probable, then, from Moseley’s work that we have already found all except three of the complete series of different types of atoms from hydrogen to lead, i.e., from 1 to 82, of which the physical world is built. From 82 to 92 comes the group of radioactive elements which are continually transmuting themselves into one another, and above 92 (uranium) it is not likely that any elements exist. That hydrogen is indeed the base of the Moseley series is rendered well-nigh certain by the following simple computation. If we write Moseley’s discovery that the square roots of the highest frequencies, , , etc., emitted by different atoms are proportional to the nuclear charges, , , etc., in the following form: