494 L. H. G R A Y 



the living cell. Change in cell morphology, and particularly changes 

 in chromosome structure, which are brought about by relatively 

 small doses of all types of ionizing radiation in both plant and animal 

 tissues, have been intensively studied. Much less is known of the 

 influence of ionizing radiations on the chemical processes that take 

 place within the living cell. Nevertheless, within limited fields of 

 study there are certain well understood broad principles, very abh' 

 expounded by Lea (1), so that a worker taking up this study may 

 reasonably expect to enlarge his knowledge of fundamental processes 

 in normal cells. In certain respects radiation may now be reckoned 

 as an agent capable of producing a controlled disturbance of vital 

 processes, and since its entry is unimpeded by cell membranes it has 

 some advantages over all chemical agents. 



In this section it will only be possible to touch upon a few aspects 

 of radiobiology that may be of interest to those not primarily con- 

 cerned in the investigation of the mode of action of ionizing radiations. 



3. Ionizing Radiations as Part of the Environment of Living 



Organisms 



Living organisms of all kinds are continually exposed to ionizing 

 radiations arising from a number of sources. These include trace 

 concentrations of radioactive substances contained within the organ- 

 ism, from the radioactive content of the earth's crust, the ocean, or 

 the atmosphere, or from cosmic radiation that enters the earth's 

 atmosphere from outer space and penetrates to great depths through 

 earth and water. 



Radioactive elements are very widely dispersed in minute amounts 

 throughout the whole of nature. They are derived from the parent elements 

 uranium and thorium, and to a lesser extent from protactinium, which are 

 present in measurable amounts in all types of rock. The y radiation from 

 this radioactive material present in the earth's crust produces about 3 ions 

 per cubic centimeter per second in air over land areas. The y ray intensity 

 over the sea is negligible by comparison, since the radioactive content of sea 

 water is in general about a thousand times smaller than that of an equal mass 

 of the earth's crust. The radioelements from the rocks very slowly pass into 

 solution in the soil moisture and then into plants. The gaseous radioele- 

 ments, radon and thoron, diffuse into the soil air, and are carried into the 

 lower atmosphere. Thoron, having a half-life of less than a minute, remains 

 very localized. Radon, which has a half-life of 3.5 days, is carried by winds 

 for immense distances over land and sea. The ionization arising from the 



