88 BIOLOGICAL EFFECTS OF RADIATION 



Paper I. In the case of X-rays and gamma rays, with which we are 

 chiefly concerned here, these conditions are always fulfilled. The inter- 

 action of matter and radiation in this case is an atomic phenomenon 

 whereby the radiation transmits some of its energy to an electron in the 

 atom and causes it to leave the atom with a high velocity and therefore a 

 certain amount of kinetic energy. This high-speed electron, then, spends 

 most of its energy in removmg electrons from those atoms in its path 

 which tend to impede its motion. The wandering electrons thus created 

 attach themselves to other atoms, forming negative ions, whereas the 

 atoms which have lost electrons remain positively charged and form the 

 positive ions. In a gas where the atoms are not closely packed, the ions 

 are relatively far apart ; on the other hand, they can move with consider- 

 able freedom. Through thermal agitation and the force of attraction 

 between positive and negative charges, ions of different polarity soon 

 come together, an interchange of electrons takes place, and neutral atoms 

 result. This process is called recombination. Ionization and recombina- 

 tion take place constantly in matter through which radiation is passing, 

 and at any one time but a very small fraction of the total number of 

 atoms present is in the ionized state. Recombination in a gas may be 

 reduced greatly, or actually prevented under certain conditions, by the 

 application of an electric field, which makes the ions of different polarity 

 move in opposite directions. In a liquid or solid the atoms are closely 

 packed, ions are nearer to one another, and motion is more difficult. 

 Recombination takes place also, and, in fact, it is much more difficult to 

 prevent it by the application of even a strong electric field. That thermal 

 agitation plays an important part in the recombination of ions may be 

 demonstrated very vividly by the following experiment : A phosphorescent 

 material exposed to cathode rays (high-speed electrons) glows brilliantly 

 at room temperature. If it is then immersed in liquid air, the phos- 

 phorence stops, but it reappears after removing the material from the 

 liquid air, when it assumes a higher temperature. (It will be recalled 

 from Darrow's discussion — Paper I — that emission of light in a phos- 

 phorescent material involves a rearrangement of electrons in the atoms.) 

 We have seen how radiation transfers some of its energy to matter 

 and how this energy is used up in the formation of ions. So far as ive 

 know at present this is the only way in which matter abstracts energy 

 from ionizing radiations. Therefore all other effects are subsequent to 

 ionization and must be closely related thereto. Since thermal agitation 

 plays an important part in the recombination of ions, it follows that a 

 good many ions of opposite polarity are brought together primarily by 

 atomic or molecular collisions. The force of attraction between positive 

 and negative ions is not sufficiently large to bring them together unless 

 the distance is very short and there are no obstacles (other molecules) in 



