EFFECTIVENESS OF X-RAY WAVE-LENGTHS 469 



of oxidation to ferric sulphate was the same. Quimby and Downes (32) 

 also have shown by another chemical method that within a very wide 

 range of \vave-lengths the amount of mercurous chloride precipitated 

 from a mixture of ammonium oxalate and mercuric chloride is propor- 

 tional to the length of exposure, other factors being the same. They 

 therefore conclude that this reaction may be useful for measuring the 

 quantity of radiation, since it is independent of wave-length effects. 



The effectiveness of X-ray wave-lengths is a problem of practical 

 importance to the radiologist. If soft rays are more potent than hard 

 rays and produce different kinds of changes in tissues, then each wave- 

 length becomes a different medicament. This view, once widely held, 

 is no longer tenable. The qualitative and quantitative effects of equal 

 doses of both hard and soft X-rays are the same. The radiotherapist, 

 therefore, now chooses that radiation w^hich can penetrate to the site to 

 be treated, knowing that the energy, in the form of short rays, actually 

 absorbed by cells at a depth will produce the same amount of change that 

 is produced by an equal dose of soft rays absorbed at the surface. 

 Whether the still shorter gamma rays of radium or X-rays of comparable 

 wave-length, generated at potentials of a million volts, will conform to 

 this rule cannot be finally determined until more accurate means of 

 measuring their intensities have been devised. 



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