The figures I have given are the times when half the parent substance has gone over into the next generation. It will be seen that the chemist is even more liberal in his allowance of longevity than was Moses with the patriarchs. It appears from the above that half of the radium in any given specimen will be transformed in about 2000 years. Half of what is left will disappear in the next 2000 years, half of that in the next 2000 and so on. The reader can figure out for himself when it will all be gone. He will then have the answer to the old Eleatic conundrum of when Achilles will overtake the tortoise. But we may say that after 100,000 years there would not be left any radium worth mentioning, or in other words practically all the radium now in existence is younger than the human race. The lead that is found in uranium and has presumably descended from uranium, behaves like other lead but is lighter. Its atomic weight is only 206, while ordinary lead weighs 207. It appears then that the same chemical element may have different atomic weights according to its ancestry, while on the other hand different chemical elements may have the same atomic weight. This would have seemed shocking heresy to the chemists of the last century, who prided themselves on the immutability of the elements and did not take into consideration their past life or heredity. The study of these radioactive elements has led to a new atomic theory. I suppose most of us in our youth used to imagine the atom as a little round hard ball, but now it is conceived as a sort of solar system with an electropositive nucleus acting as the sun and negative electrons revolving around it like the planets. The number of free positive electrons in the nucleus varies from one in hydrogen to 92 in uranium. This leaves room for 92 possible elements and of these all but six are more or less certainly known and definitely placed in the scheme. The atom of uranium, weighing 238 times the atom of hydrogen, is the heaviest known and therefore the ultimate limit of the elements, though it is possible that elements may be found beyond it just as the planet Neptune was discovered outside the orbit of Uranus. Considering the position of uranium and its numerous progeny as mentioned above, it is quite appropriate that this element should bear the name of the father of all the gods.