JoLY — Ou the Local Apijlication of Radium in Therapeutics. 291 



morbid tissues. It also fails to provide cross-radiatiou, a condition which has 

 been found to give tlie best results. In order to overcome partly the last 

 objection, two, and I believe as many as three, tubes have been used in the 

 ease of large tumours. 



We cau estimate the loss involved in screening, as above described, by 

 reference to recent measurements made by Rutherford and H. Eichardson.' 



When radium, or its emanation, is used, the gamma rays concerned in 

 the treatment are those from Ra B and Ra C. From the first substance three 

 types of rays proceed — a very soft type, for which the absorption coefficient 

 in aluminium is 230 (cm.)"' ; a soft type for which the coefficient is 40 ; and a 

 harder type for whicli the coefficient is 0'51. The rays fromEa C are homo- 

 geneous, and for them tlie coefficient (i is 0-115 (cm.)"' in aluminium. 



The softest rays emitted by Ra B are cut off by almost any envelope we 

 clioose to employ ; they are, in fact, but little more penetrating than alpha 

 rays. The second type will be reduced to ten per cent, of their original value 

 by as little as one millimetre of aluminium. In this calculation we use the 

 equation I = I^e'^^, where x is the tliickness of aluminium, and ya has the 

 value 40. The hardest type of rays from Ra B will be transmitted in some 

 degree, even through 0'3 cms. of lead. In this substance the coefficient for 

 these rays has the value of 2-8 (cm.)"'. These rays must, therefore, be taken 

 into account in considering the therapeutic application of radium or its 

 emanation, and, along with these, the rays of Ra 0, for wliich the coefficient 

 in lead is 0'50 (cm.)-'. 



The curves determined by Rutherford and Richardson (loc. cit.) are 

 reproduced here. The vertical ordinates represent tlie ionisation effected 

 in the air of an electroscope after the rays have passed through the thickness 

 of lead plotted along the horizontal axis. The two types of soft rays from 

 Ra B are, however, prevented from entering the electroscope by a suitable 

 screen of lead. The beta radiation is deflected by a strong magnetic field. 



Curve B defines the ionisation due to Ra B ; curve C that due to Ra C. 

 Curve A is the summation of the effects defined by the two curves C and B. 

 We are only concerned with the summation curve in the remarks now to 

 be made. 



We can, from this curve, readily determine the loss due to a screen of 

 o mms. of lead, sucli as is ordinarily placed around the tubes used in tiiera- 

 peutics. This we do by direct measurement of the areas enclosed by the 

 curve and the vertical and horizontal ordinates concerned. The valuable part 

 of the curve not being exponential, we cannot proceed by integration. 



* ' "I'liil. Mag,,'' May, 1913. 



2z 2 



