Living organisms breathe in carbon (in the form of carbon dioxide) from the air. Most of this carbon is ordinary stable carbon¹²; a tiny fraction is radioactive carbon¹⁴. The organism is unable to distinguish between the two isotopes, and takes in carbon¹⁴ in the same ratio to carbon¹² as exists in the atmosphere. This ratio persists throughout the organism’s lifetime, but when the organism dies and no more carbon is assimilated, the ratio begins to decrease as the carbon¹⁴ nuclei gradually disintegrate. By observing the ratio of carbon¹⁴ to carbon¹² in fossil remains and other archeological objects, the date at which death occurred can be calculated. In this way the age of ancient Egyptian mummies has been found, and it has been shown that some sequoia wood is more than 1,500 years old. By measuring the carbon¹⁴ in trees that were killed by the last advance of glaciation, and looking into other remains of life from the last ice age, it has been possible to show that this last ice age occurred only 10,000 years ago—instead of 20,000 years, as had been previously believed. Carbon¹⁴-dating has therefore thoroughly revised our ideas about the rapidity with which the empires known to history have emerged from the most primitive conditions. A crucial part of the argument is that isotopes of the same element are chemically indistinguishable. An alternative reaction which may occur when neutrons strike nitrogen, is N¹⁴ + neutron → carbon¹² + H³ (7 protons) (6 protons) (1 proton) (7 neutrons) (6 neutrons) (2 neutrons) H³, triton, is also radioactive, undergoing a beta decay to become He³ (2 protons and 1 neutron) with a half-life of 12.25 years. Tritons too can be used for dating old objects—for example, old wine. The water in the wine cannot be replenished with cosmic-ray tritons after the wine has been bottled. Thus fifty per cent of the tritons disappear every 12.25 years. We have here two examples of nuclear reactions induced by neutron bombardment. Recalling the disadvantages of charged particles as nuclear projectiles for alchemists, it must surely seem that neutrons would be ideal for this purpose. Being chargeless, they are neither electrically repelled by the nuclei nor constantly slowed down by energy-losing collisions with the electrons. The fate of almost every neutron moving in a large piece of matter is eventual collision with a nucleus.[7] Neutrons are ideal nuclear projectiles, except for one thing: they are hard to get. Protons and alpha particles are found abundantly in nature as the nuclei of hydrogen and helium atoms. Neutrons, however, are not found in nature, and in the past have been made in nuclear reactions that were themselves initiated by charged particles. For example, He⁴ + beryllium⁹ → C¹² + neutron (2 protons) (4 protons) (6 protons) (2 neutrons) (5 neutrons) (6 neutrons)