We have said that two charged particles having the same energy, produce the same total number of ionizations. There is an important respect, however, in which charged particles of the same energy may differ. That is, in the density of ionization along their paths. In particular, the more slowly the particle is moving and the greater its charge, the more ionization and damage it will produce in a given distance. At the same time it will lose energy at a greater rate. If we compare two charged particles of the same energy plowing into matter, the one which leaves the deeper furrow will be stopped more quickly. For a greater charge it is easy to understand that the electrical interaction is increased and hence each atomic electron is more strongly disturbed. If, on the other hand, the particle moves more slowly (which is usually the case if it is heavy) it spends a longer time in the neighborhood of the atomic electrons. The electrical interaction thus has a longer duration and is more effective in ejecting an electron. For this same reason the density of ionization along the path of a particular charged particle should tend to become greater and greater as the particle slows down. Actually this tendency is opposed in the case of a fission fragment by the increased likelihood of the particle’s picking up electrons and reducing its charge. As a result, the ionization density for these fragments is rather uniform. If a heavily charged, slow particle moves through matter it leaves so many disturbed and disrupted molecules behind that now these molecules may react with each other. Therefore heavy ionization may lead to peculiar effects. Nevertheless all ionizing particles give rise to roughly similar chemical change and destruction. Except for the beta rays, all the charged particles are very heavy compared to the electron. Consequently, as they move through matter and interact with the atomic electrons, their paths are not perceptibly deflected from the original direction. The beta rays, on the other hand, having the same weight as the atomic electrons, are appreciably affected by their encounters and are frequently forced to change direction. Their paths are thus winding and random. Because the beta ray does not travel in a straight line, its ability to penetrate matter must not be measured by its total path length. As a rule of thumb, the _range_ of a beta particle, being the distance it travels along the line of its original direction, is about one half of its total path length. For heavier charged particles, however, no distinction need be made between range and actual distance traveled.