the ear due to the penetrating action of X rays. 

 Such photographs are known as radiographs. 



The ionization method is based on the ability 

 of Xrays to cause air and other gases to become 

 conductors by imparting positive or negative 

 charges to ions of uncharged gas molecules. 

 The amount of current passing through the ion- 

 ized gas with a given difference in potential is 

 proportional to the intensity of the X rays causing 

 the ionization. Special ionizing chambers which 

 are used to measure the degree of ionization of 

 the gas can serve as dosimeters for quantitative 

 measurement of X rays. 



In their optical properties X rays show all of 

 the characteristics of light rays. They can be 

 refracted and reflected. They exhibit the phe- 

 nomena of polarization and diffraction. The 

 ability of X rays to penetrate deep into tissues 

 and to induce various changes there is of partic- 

 ular significance in biology. It should be noted 

 that only rays which are absorbed exert any 

 effect. Rays which penetrate feebly or those 

 which are completely reflected have no action. 

 The ability to penetrate increases as the wave 

 length shortens. 



During the first year of discovery of X rays 

 people working with them experienced serious 

 inflammations of the skin accompanied by loss 

 of hair. It soon became apparent that the action 

 of the rays is not limited to their effect on the 

 skin, but that all the internal organs of men and 

 animals are affected if the amounts of exposure 

 are sufficiently great. Radiation therapy is 

 based upon this phenomenon. 



In 1903 Heinecke* amazed everyone by his 

 discovery that mice and guinea pigs can be 

 killed by X rays. Autopsy of animals killed in 

 this manner showed that their spleens were 

 small and dark; histological examination re- 

 vealed an extraordinary increase in the amount 

 of pigment and the disappearance of follicles 

 and cellular elements of the spleen. Similar 

 changes took place in the lymph nodes, bone 

 marrow, etc. In cases of sub -lethal dosage a 

 regeneration of lymphatic tissue commences 

 after several days. However, a discussion of 

 this aspect of X ray action is outside the scope 

 of this book; those interested in this phenomenon 

 are referred to the book written by Nemenov in 

 1926 [1933?]. 2 



The Measurement of Energy. Before pro- 

 ceeding further some concept of the units of 

 measurement of X rays should be established. 



This is particularly imperative because the use 

 of various units of measurement by the numer- 

 ous workers in this field increases the difficulty 

 of comparing the results obtained in the radia- 

 tion of various plants and animals. 



The most widespread unit of measurement, 

 up to recent years, was a dosage bearing the 

 abbreviation HED [Skin Erythema Dose], This 

 dose is equivalent to that amount of radiation 

 which results in a reddening and peeling of the 

 skin in man ten days after exposure to radia- 

 tion. But this measure, like most biological 

 units, is too inaccurate since the indicated skin 

 symptoms vary with the age, physical condition, 

 and skin pigmentation of a given patient. Clark 3 

 who examined many patients in various clinics, 

 has shown that the erythemic dose varies from 

 400 r to 1200 r (roentgen units) depending on 

 individual differences, and that the average 

 dose equals about 840 r. 



Many German and French investigators 

 used the Holtzknecht unit (H) (equal to about 

 1/5 of the erythemic dose) to measure X 

 rays. 



This latter unit was abandoned in favor of a 

 new international unit r (roentgen unit) which 

 corresponds to the amount of energy required 

 to produce, as a result of complete ionization 

 in air, at zero degrees centigrade and normal 

 atmospheric pressure, charges of one electro- 

 static unit per cubic centimeter of the irra- 

 diated object. 4 



Dosimeters used for quantitative determina- 

 tion of X rays give the measurement in roentgen 

 units. The majority of investigators, however, 

 are not content to indicate the amount of X rays 

 in roentgen units; they also indicate the source 

 of radiation, voltage, amperage, distance of 

 the object from the tube, duration of exposure, 

 and nature (or absence) of the filters. 



Filters are very thin plates (of aluminum, 

 copper, etc. ) and serve to dissipate the X 

 rays of specific wave lengths. This makes it 

 possible to achieve some uniformity [ of wave 

 lengths] in the beam which irradiates the 

 obj ect . 



^o 1926 reference given; see 1933 reference. 



'Reference not given. Can be found in Discussion 

 to Packard, C. 1934. Biological dosimeters in radi- 

 ology. Cold Spring Harbor Symposia Quant. Biol . 2, 

 264-273. 



^Editor's note: Not the currently accepted definition 

 in general usage. 



