XV. ELECTRONS, NEUTRONS, AND ALPHA PARTICLES 537 



suits when small organisms having a large ratio of surface to volume 

 are irradiated. It may be minimized by dissolving the radon in 

 2% gelatin solution instead of in water. The RaA and RaC atoms 

 apparent!}'' then become attached to protein molecules and are much 

 less mobile. In the experiments of Kotval and Gray (2) shoots 

 bearing inflorescences, stripped of their bracts, sepals, and petals 

 were immersed in radon solution. Even with the modification of dis- 

 solving the radon in 2% gelatin the absorbed activity on the surface 

 of the anthers increased the mean dose to the microspores by 10%. 



When tadpoles were allowed to swim freely in radon solution the 

 attainment of an equilibrium concentration throughout the animal 

 was aided by imbibition and a blood circulation, so that the time 

 taken to reach half equilibrium concentration was not much greater 

 than in the case of roots. If diffusion alone were operative the time 

 would be proportional to the square of the linear dimensions. In the 

 case of larger animals the uniformity of radon distribution should be 

 carefuU}^ investigated. Radon is very much more soluble in many 

 organic substances than in water. Radioautographs showed slight 

 evidence of inhomogeneity in the distribution of radon in the tadpole. 

 Studies on radioactive xenon (99), which probably closely resembles 

 radon as regards solubility, have shown verj^ large variations indeed 

 in concentration in different organs of the rat. In view, however, of 

 the importance of comparing wherever possible the efficiency of high 

 and low" ion density radiation it is worth while to take some trouble 

 in the design of an experiment so as to permit the use of a radiation, 

 more especiallj^ in laboratories not provided with neutron sources. 



D. MEASUREIVIENT OF IONIZING RADIATION 



In the preceding sections numerous instances have been given of 

 the way in which the same total dissipation of energy in tissue by 

 ionizing radiations may affect vital processes to a different extent ac- 

 cording to the distribution of ions within the cell, the duration of ir- 

 radiation, and other factors. These differences were only brought 

 to light after means had been devised for estimating the total energy 

 dissipated by the ionizing particles in all forms — ionization, excitation, 

 chemical change, and heat — per unit mass of tissue. 



The quantity of energy with which we are concerned is 93 ergs or 

 2.2 X 10~^ cal. per gram of tissue per roentgen. For technical reasons 

 it would only be possible to attempt to measure this energy calori- 



