1138 BIOLOGICAL EFFECTS OF RADIATION 



in sensitivity. Similar curves are produced, of course, by such a diffuse 

 lethal factor as temperature. An interesting physical interpretation 

 of effects of this nature is the assumption that any organism or cell will 

 die as soon as it has received the required, or critical, number of 

 X-ray quanta; that is, the probability feature applies to the number of 

 quantum hits received, and not to the variability of the biological 

 material. 



The further suggestion was made from the work of Crowther with 

 Colpidium that a hit on a particular, sensitive spot of the protoplasm is 

 necessary to cause death. There has been much discussion of this view 

 and modifications of it both in regard to the size and the distribution of 

 the sensitive constituents and in respect to the effects of quanta of differ- 

 ent energy content. Holweck and Lacassagne (78, 88) and Glocker (65), 

 among others, are prominently associated with work done in this field. 

 An extended discussion, with literature, will be found in a recent paper 

 of Lacassagne (89), and to this the reader is referred, likewise to the 

 discussion by Gowen (Paper XLIII) in this work. Wyckoff has also 

 emphasized the physical side, and some of his results will be given by 

 way of illustration. 



In two papers Wyckoff (164, 165) has reported quantitative studies on 

 the lethal effects of X-rays on bacteria. The first paper is concerned 

 with effect of soft X-rays and the second with X-rays of different wave- 

 lengths. The bacteria were exposed on agar surfaces. Counts were 

 made after incubation on equal exposed and adjacent control areas. In 

 the first experiments, B. coli and B. aertrycke were used, the assumption 

 being that these organisms give a distribution in single cells, an assump- 

 tion which appears to the reviewer less real than ideal. Soft X-rays 

 were supplied from the general radiation of a tungsten tube operated at 

 12-kv. peak and 8 ma., and also by the K radiation of copper. Intensity 

 measurements were made. Survival values were plotted against time. 

 The results indicate that there is no significant difference in the sensitive- 

 ness of the two species of bacteria. The killing effect is semilogarith- 

 mically linear. Basing the conclusion on a mathematical-physical 

 analysis, it is concluded that death is caused by the absorption of a single 

 X-ray quantum of energy. Moreover, the ratio of quanta absorbed to 

 killing is about 20/1. It would appear that the "sensitive cell con- 

 stituents" must have a volume less than 0.06 of the entire cell. Effects 

 other than lethal ones were not given special consideration. 



MISCELLANEOUS CONSIDERATIONS 



Apparatus and Procedures. — A general discussion of radiation tech- 

 niques and procedures has no place in this paper, but brief indications 

 regarding special applications may be appropriately given. It is clear 

 that the source of radiation and the optical or filter systems or other 



