252 proceedings: philosophical society 



have frequently led to loss of life. Cancer has also resulted from 

 X-ray burns. The latter class comprises the destruction of the lympho- 

 cytes and of the tissue forming them, with consequent deleterious effect 

 on the bodil}^ economy. The former class of lesion is the more im- 

 portant practically. 



The absorption coefficient of a substance is defined as the quantity 

 X in the expression : 



for the intensity/ of a homogeneous beam of X-rays at a point d centi- 

 meters below the surface. X increases with increasing wave length. X-ray 

 absorption is independent of the state of chemical combination of the 

 absorbing elements. Weight for weight, elements of high atomic weight 

 are more efficient absorbers than those of low atomic weight. 



Of the elements of high atomic weight lead is, for practical reasons, 

 always used for protection from X-rays. It is used in the metallic 

 state and also as the oxide in glass and rubber. Silk loaded with a lead 

 salt has also been used. ' 



There is no general agreement as to the thickness of lead necessary 

 to give adequate protection. The amount, of course, would depend 

 upon the quantity and quality of the rays against which protection is 

 necessary. For long exposures the German Rontgen Society advocates 

 one of the following: metallic lead, 2 mm. thick; lead rubber, 8 mm. 

 thick; lead glass, 10 to 20 mm. thick. 



In estimating the protection offered by j^rotective materials it is much 

 more convenient to ascertain the amount of lead contained in the mate- 

 rial than to measure the absorption coefficient. This latter varies 

 with the hardness of the rays used and with the constitution of the beam. 

 Lead is a standard and easil}^ reproducible substance and a knowledge 

 of the lead content of a piece of protective material will enable the user 

 to estimate easily the protection offered at any time when investigations 

 may have rendered our knowledge of the necessary protection more 

 exact. 



The absorption of a piece of protective material is due principally to 

 the lead content. The absorption of the remaining constituents is 

 equivalent, for practical purposes, to that of a certain small additional 

 thickness of lead. The whole piece, therefore, is equivalent to a certain 

 thickness of lead. This statement has been verified experimentally. 

 The simplest method for estimating the protection, and the one used at 

 the Bureau of Standards, is to place the substance to be measured on an 

 X-ray plate- beside a series of thicknesses of lead. The density of the 

 plate under the material is then matched with the density of one of the 

 thicknesses of lead. This thickness is termed "the equivalent thick- 

 ness of lead" for the material. It has been shown experimentally that 

 it is independent of all conditions and is the same if a fluorescent screen 

 is used instead of an X-ray plate. The ratio of the equivalent thick- 

 ness of lead to the thickness of the material is termed the "protective 

 coefficient" of the material. 



