Roentgen and Becquerel were exploring what they thought was an untrodden path in the forest and came upon a new road that led their successors to the very citadel of the material universe. Young Enrico Fermi was curious to find out what would happen if he fired a neutron into the nucleus of uranium, hoping only to create a heavier isotope of uranium, or at best a new element. His rather modest goal led five years later to the fission of uranium, and in another six years to the atomic bomb. Yet, as we have seen, in both fission and fusion only a very small fraction of the mass of the protons and neutrons in the nuclei of the elements used is liberated in the form of energy, while 99.3 to 99.9 per cent of their substance remains in the form of matter. We know of no process in nature which converts 100 per cent of the matter in protons and neutrons into energy, but scientists are already talking about finding means for bringing about such a conversion. They are seeking clues for such a process in the mysterious cosmic rays that bombard the earth from outer space with energies billions of times greater than those released by fission or fusion, great enough to smash atoms of oxygen or nitrogen, or whatever other atoms they happen to hit in the upper atmosphere, into their component protons and neutrons. Luckily, their number is small and most of their energy is spent long before they reach sea level. But we have already learned how to create secondary cosmic-ray particles of relatively low energies (350,000,000 electron-volts) with our giant cyclotrons. The creation of these particles, known as mesons, which are believed to be the cosmic cement responsible for the nuclear forces, represents the actual conversion of energy into matter. This is the exact reverse of the process taking place in fission and fusion, in which, as we have seen, matter is converted into energy. And we are now about to complete multibillion-volt atom-smashers that will hurl atomic bullets of energies of from three to ten billion volts at the nuclei of atoms. With these gigantic machines, known as the cosmotron (at the Brookhaven National Laboratory of the Atomic Energy Commission) and the bevatron (at the University of California), we shall be able to smash nuclei into their individual component protons and neutrons and thus get a much more intimate glimpse of the forces that hold the nuclei together. What is more, instead of creating only mesons, particles with only 300 electron masses, we shall be able for the first time to convert energy into protons and neutrons, duplicating, as far as is known, an act of creation that has not taken place since the beginning of the universe. Man at last will be creating the very building blocks out of which the universe is made, as well as the cosmic cement that holds them together.