XIV. X RAYS AND X IRRADIATION 473 



were randomly destroying the blood-forming cells of the body and 

 thus lowering the number of leucocytes available for opposing the 

 disease organisms. 



Much interest has been attached recently to the combination ef- 

 fects of X-ray irradiation with other types of energy, as ultraviolet 

 and infrared (33-35,50-53). These combination effects have been 

 studied quantitatively in Tradescantia and Drosophila. Other forms 

 of energy combined with that of X-ray irradiation lead to results that 

 are pronouncedly different from those when X rays are used alone. 



These illustrations of the use of X rays in experimental work de- 

 pend for their interpretation upon the accurate measurement of ir- 

 radiation dosages and quantitative estimation of the biological effects. 

 The gathering of the data, their statistical analysis, and their inter- 

 pretation form the keynote to research advance. 



F. METHODS OF MEASURING BIOLOGICAL EFFECTS 



OF X RAYS 



I. Virus Materials 



Some plant viruses may be irradiated either dry, in water, or in 

 physiological salt solutions. If the,v are irradiated dry on cello- 

 phane, the layer (^f material is so thin that the difference in absorption 

 between the surface and the cellophane may be so slight that no cor- 

 rection of the dose for differential absorption need be made. On the 

 other hand, when irradiated in solutions, the absorbing power of the 

 solution must be taken into consideration and, if possible, the depth 

 of the solution made so small that its absorption does not become 

 great. The solution for carrying the treated material should be one 

 in which the radiation will not produce a toxic substance. There are 

 some chemicals that on irradiation materially affect bacteria and 

 viruses. In fact, water itself may release oxygen, which can become a 

 source of error in the experiment. 



After irradiation and proper measurement of the dose as it is re- 

 ceived by the organism, it is necessary to separate the individual 

 bacteria or virus particles so that a count can be made of the original 

 number and also the number that survive the X-ray treatment. 

 Bacterial species should be chosen that do not form chains or clump 

 and thus completel}^ vitiate the X-ray experiment. Similarly, virus 

 particles must be capable of being distributed separately over the test 



