28 PHYSICAL MEASUEEMENT OF X-RAYS. — BRONSON. 



produced in air surrounding the pastille during normal dose 

 is 4N or about 6.4 XlO^^ In measuring this dose the pastille 

 is ordinarily placed half way between the anticathode and 

 the skin. Therefore the number of ions per c.c. produced 

 near the surface of the skin during an epilation dose would 

 be 1.6 XIO^^ This vahie is apparently very much smaller 

 than that obtained by Dr. Szilard (loc. cit.), although there 

 is some confusion in the part of his paper dealing with this 

 calculation. A small part of this difference is due to the 

 small value, 3.4 XlO"^'' which he used for e, but the chief 

 difference is due to the nature of the testing vessels used in 

 the two experiments. 



The vessel used by Dr. Szilard had a volume of 1 c.c. 

 and was lined with lead. In a vessel of this kind the ioniza- 

 tion due to secondary rays would be very large. Some 

 of the secondary rays are very easily absorbed, but produce 

 an intense ionization for a millimeter or two in air, so that 

 their relative effect is especially great in a small vessel. 

 The effect is also much greater with a lead than an aluminum 

 vessel. Even in the large vessel used in the present investi- 

 gation, the ionization was doubled by covering the plate C 

 with lead foil. In the smaller vessel the ionization caused 

 by the secondary rays might be several times as large as that 

 due to the X-rays themselves. That the effect of the second- 

 ary rays from the aluminum plate C was small, was shown 

 by covering C with a sheet of wet tissue paper, which reduced 

 the ionization current less than 10%. 



There is a still greater objection to using lead instead of 

 aluminum for the interior of the testing vessel; namely the 

 fact that the relative amount of secondary ionization depends 

 on the hardness of the rays. The extra ionization in the 

 testing vessel also requires the use of a higher voltage in 

 order to prevent recombination. 



Summary. 

 1. It has been shown that the action of Roentgen rays 

 on a photographic plate and on a Sabouraud pastille is 



