116 BIOLOGICAL EFFECTS OF RADIATION 



organism, having a density about 800 times greater than that of atmos- 

 pheric air, will also disturb the ionization at the point where it is placed. 

 However, the disturbance in this case must be considered from the point 

 of view of the ionization produced in the solid material itself. As in the 

 case of the closed ionization chamber, it will be seen by analogous reason- 

 ing that, under certain conditions, compensation may occur, whereby 

 the number of ions liberated per second per cubic centimeter of the bio- 

 logical object is higher than the number of ions Hberated per second per 

 cubic centimeter of atmospheric air at the same point, in the ratio of the 

 densities of the two materials. That is, the ionization per cubic centi- 

 meter of tissue will be roughly 800 times greater than that per cubic 

 centimeter of atmospheric air at the same point. As one might expect, 

 the compensation or balance just spoken of depends on the quaUty of 

 the radiation and on the size of the biological object (including container, 

 holder, etc.) placed in the beam. For very small objects it obtains 

 reasonably well in the usual range of wave-lengths employed in practice. 

 In such cases it is sufficient to measure the ionization in air in order to 

 deduce that in the tissues. This can be done for the usual qualities of 

 X-rays by means of either a standard chamber or a properly calibrated 

 "air-wall" chamber. When this procedure is not justified, the problem 

 of determining, or even estimating, tissue ionization becomes more 

 involved. Realizing that the principal comphcations arise from the 

 great difference in density between living tissue and atmospheric air, 

 one might attempt to solve the problem by measuring the ionization in 

 an organic material of density comparable with that of living matter. 

 All efforts made in this direction so far have not been successful, but the 

 work should be continued. For the present, however, being hmited to 

 measurements made in (gaseous) air, great care should be exercised in 

 planning biological experiments, to make sure that the conditions under 

 which the living organism is irradiated, permit a reasonably close estimate 

 of the ionization produced therein. 



The selection of the proper conditions of irradiation depends on many 

 factors and no general rules can be given here. The following suggestions 

 will be found helpful: Experiments may be undertaken for the main 

 purpose of describing hitherto unknown effects of radiation. These are 

 exploratory experiments, in which normally no attempt is made to 

 establish quantitative relationships. In such cases the choice of experi- 

 mental conditions is not important. However, it is necessary to give 

 sufficient information about the experimental set-up and technique to 

 enable someone else to duplicate the results. The primary factors, 

 such as voltage, milliamperes, filter, distance, etc., stated in the pre- 

 ceding paper (L. S. Taylor) suffice for this purpose. If all these factors 

 are carefully determined and described, they are sufficient, also, to 



