Spectrum of Penetrating 7 Rays from Radium B and C. 273 



simply shown that the frequency v is connected with fi by 

 the formula v = k 1 / J L—i where k x is a constant. 



It is of interest to examine whether this formula holds 

 approximately for the penetrating radiations from radium B 

 and radium C. The soft 7 rays for which //,=40 give rise 

 to two strong lines at 10° and 12° or a mean reflexion angle 

 of 11°. It follows from the above formula, that the strong 

 line or lines due to the penetrating radiations from radium B, 

 for which yu, = *51 in aluminium, should give a mean reflexion 

 angle of 1° 54'. The actual reflexion angle observed for the 

 strongest line is 1 40 about — a fair agreement. 



Similarly, the penetrating rays from radium 0, for which 

 fi= '115 in aluminium, should give an average reflexion angle 

 of 63'. This is in good accord with the average 58' of the 

 reflexion angles of the stronger lines, viz. 43', 60', 70'. The 

 results thus show that this simple method of calculating 

 frequencies from absorption coefficients hold approximately 

 over a very wide range. It is possible, for example, by this 

 method to calculate approximately the mean position of the 

 strongest line or group of strong lines of the various types 

 of 7 rays from radioactive substances whose absorption 

 coefficients have been given by Rutherford and Richardson. 



Hardening ofy Rays. 



It has long been known that the 7 radiation from radium C 

 becomes more penetrating for aluminium after it has passed 

 through a centimetre or more of lead. This hardening effect 

 has been recently examined in detail by Oba *, who has 

 shown that after passing through a considerable thickness of 

 lead the radiations become permanently more penetrating 

 for aluminium. He has explained these results on the 

 supposition that the spectrum of radium C is complex. We 

 have seen that the main reflexion lines from radium C are at 

 44', 1° 0', and 1° 10'. It is probable that the radiations of 

 frequencies corresponding to the two latter are absorbed 

 more rapidly than that corresponding to the former. On 

 such a view it would be anticipated that the penetrating- 

 power of the 7 radiations from radium C would increase 

 until the softer rays are all absorbed, and then remain 

 constant, corresponding to a homogeneous radiation reflected 

 at 44' from rock-salt. 



Our best thanks are due to Dr. E. S. Kitchen for a large 

 number of crystals of rock-salt. 



* S. Oba, Phil. Mag. April 1914, p. 601. 

 Phil Mag. S. 6. Vol. 28. No. 164. Aug. 1914. T 



