In summary, all particles, charged or not, have a similar action on matter. Directly or indirectly, they produce excited atoms, molecules, and ion pairs. These processes always occur in practically the same proportions, and therefore the number of ion pairs formed can be used as a measure of the radiation effects. The more ion pairs produced in living matter, the greater the extent of biological damage. For this reason it is customary to describe radiation effects in terms of the number of ion pairs created per gram of living tissue in various parts of the body. Since each ion pair corresponds to an energy transfer of about 32 electron-volts, an alternative description may be given in terms of the amount of energy deposited. The unit in common usage for this purpose is the _roentgen_, which means specifically an energy equivalent to lifting the body (in which the radiation is deposited) by one twenty-fifth of an inch. This is equivalent to about 60 million million ion pairs in each ounce. It is less exact but more significant to say that one roentgen deposits in a cell of our body a few thousand ion pairs. Of course the amount of ionization within individual cells is not a quantity that is easily measured. What one usually knows instead, is the roentgen dosage to a piece of tissue, which consists of many cells. If the charged particles inducing the ionization are electrons (as they are when the primary radiation is a beta ray or a gamma ray), the ionization will be distributed more or less uniformly among the cells in the affected neighborhood. If the charged particle is heavy—a proton or an alpha ray—the density of ionization which it produces is much greater, so that some cells receive a good many more ion pairs, while others nearby may receive none. For this reason it is sometimes important to specify not just how many roentgens the tissue has been exposed to, but also which kind of radiation has been responsible. In a later chapter we shall discuss the biological effects of various amounts of radiation. We may mention here, however, that 1000 roentgens of X-rays or gamma rays delivered more or less uniformly over the whole body of a human being in a time less than a few hours or so, will lead to almost certain death. And it is a remarkable fact that nature has not provided us with a warning. Radiation does not hurt. The greater is the need that we understand this process which affects our well-being but not our senses. CHAPTER IX The Test Testing of atomic explosives is usually carried out in beautiful surroundings. There is a good reason for this: the radioactive fallout.