JoLY — Radio-Therapy : its Scientific Basis and its Teaching. 499 



The faster electrons probably penetrate a couple of centimetres in soft tissues, 

 their energy dying out in the creation of ions and secondary y rays. The 

 latter are again re-converted to !^ radiations, which again take up the work of 

 ionisation. Thus the whole of their energy, or the greater part of it, is, 

 probablj^ ultimately spent in the work of ionisation: in other words, on 

 work which is capable of seriously modifying the chemical and molecular 

 processes progressing in the medium. 



(3) Tliere emerge, also, from the tube the y rays of E.a B and Ra C. The 

 latter enormously predominate, most of the rays of Ra B being sutKeiently 

 soft to be absorbed in the walls of the tubes. These rays, as we liave seen, 

 move with certain definite quanta of energy, or in integrals of a certain 

 quantum in each case : in short, in trains of rays. Wherever they traverse 

 atoms they give rise to j3 rays. Some of these, taking up the whole energy 

 of a wave-train, move with velocities similar to the most penetrating primary 

 /3 rays given out by the parent radioactive atoms. This is the inner history 

 of the events leading to the ionisation of the medium according to recent 

 views. 



The number of ions which these rays can generate in air lias been 

 computed.' In reckoning the number of ions two count for one, as each 

 electron separated involves the formation of both a + and a - ion. The 

 numbers given refer, therefore, to pairs of ions. 



The quantities of tlie substances RaB and RaO used in the estimates are 

 those which will be in equilibrium witli one gram of elemental radium or 

 with one curie of emanation. These substances alone concern the surgeon 

 when he applies radioactive treatment by usage of the emanation. The rays 

 are supposed to act for one second of time. 



From /3 rays of Ra B . . 0-325 x 10'^ 

 „ ^ „ RaC . . 0'64 X 10'^ 



RaB , . 0-084 x 10>^ 

 RaC . . 1-134 X 10^^ 



Total, 2-183 x 10'^ 



Now it is of interest to estimate what these numbers would represent 

 in the therapeutic use of these radiations in body tissues, on the assumption 

 that the energy is in a similar degree expended on ionisation — an assumption 

 which may approximate to the facts, seeing that the mere state of aggrega- 

 tion — solid, liquid, or gaseous — should not much affect the results. I take 



' Moseley and Robinson: Phil. Mag., vol. xxviii, Sept., 1914. 

 SCIENT. PROC. K.D.S., VOL. .X.IV., NO. XXXVII. 4 K 



