by a Particles from Radium Emanation. 917 



The operation of introducing the emanation into this tube 

 was exactly as described previously, but the whole tube was, 

 in this case, placed between the poles of a large electro- 

 magnet giving a field of about 3000 gauss, to prevent any 

 rays entering the electroscope. 



The rise of 7 radiation from zero time was taken with 

 various absorbing screens of aluminium placed over the 

 examination-tube and between the pole-pieces of the magnet, 

 as shown in fig. 4. 



For the same reasons as given before (in Part I.), the 

 amount of radiation excited by a particles in any element is 

 obtained from the difference between the activity in divisions 

 per minute at zero time, when (1) the element is the target, 

 (2) rice-paper is the target bombarded by the ol particles. 

 This eliminates any errors due to the fastest rays which 

 may escape the action of the magnetic field and enter the 

 electroscope. It does not avoid the effect on the form of 

 the rise-curve discussed in Part I. of this paper. 



In this way the quantity of radiation, in divisions per 

 minute per milligram of emanation used, excited in the 

 element under consideration is obtained. This quantity was 

 measured through various thicknesses of aluminium, and the 

 quality of the radiation deduced from the usual ionization- 

 thickness curve. 



The electroscope base was covered by a thin sheet of 

 aluminium, and the total aluminium equivalent of the electro- 

 scope base, mica, and air-space between the examination- 

 tube and the electroscope was 0*12 mm. 



The elements investigated were Pt, Au, Pb, and Sn. 

 The Pt, Au, and Sn targets were each equivalent to 3 cm.- 

 of air in a-particle stopping-power. Difficulties were en- 

 countered with the lead foil, as it is not possible to roll this 

 metal down to the same extent as the metals mentioned above. 

 The best sheets obtainable weighed 38 milligrams per sq. cm. 

 Thus beyond the extreme range of the a particles there was 

 a thickness of lead equivalent to 0*13 mm. of aluminium, 

 which brought the total minimum absorption thickness to an 

 equivalent (in aluminium) of 0*25 mm. 



The absorption curves (in aluminium) for the Au and Sn 

 radiations are shown in fig. 5. The gold curve is of the 

 same form as the platinum and lead curve, and is probably 

 the typical heavy element curve. It is quite obvious that 

 there are two well-defined radiations present : (1) a hard 

 radiation, (2) a soft radiation. The smooth absorption 

 curves for each of the elements investigated are analysed in 



