814 Sir E. Rutherford and Dr. J. Chadwick on the 



screen of aluminium of stopping-power equivalent to 3*5 cm. 

 of air is placed over the source and the air replaced by 

 oxygen. It is seen that the particles liberated from 

 aluminium are able to penetrate a much greater distance of 

 matter than those from nitrogen. The number of scintil- 

 lations becomes so small for large absorptions that it is 

 difficult to fix the limiting range with accuracy. 



Effect of Velocity of a Particles. 



A series of experiments was made to find how the number 

 and range of the particles were affected by change in the 

 velocity of the incident « particles. For this purpose the 

 following sources were employed : — 



(1) Deposit of thorium C obtained by immersing one side 

 of a nickel disk in a strong solution of radiothorium *. This 

 source gives complex a rays, viz., 2/3 of range S'6 cm. and 

 1/3 of range 5*0 cm. The 7-ray activity of this source with 

 thorium and D in equilibrium was measured in terms of 

 milligrams of radium. Recently some experiments have been 

 made in this laboratory by Professor Schlundt and Mr. 

 Shenstone to compare the number of « particles of long range 

 (8'6 cm.) emitted from Th C and D with the corresponding 

 number from Ra B and C of equal 7-ray activity. Under the 

 conditions of measurement in our experiments, the number of 

 long-range particles per milligram of activity of ThC is taken 

 as 70 per cent, of the corresponding number of « particles of 

 range 7 cm. per milligram of radium 0. In order to make 

 the results for thorium C and radium C comparable, it is 

 consequently necessary to multiply the numbers of scintil- 

 lations found in the ThC experiments by 1*4. This has been 

 done in all direct comparisons. 



(2) A source of radium B + C giving a rays of 7 cm., 

 obtained by exposure of a brass disk to the emanation of 

 radium. 



(3) Sources of a rays of ranges 6 and 4*9 cm. of air were 

 got by placing silver or gold foil of stopping-power 1 cm. 

 and 2*1 cm. respectively over a source of radium B + C. 

 Previous experiments had shown that neither silver nor gold 

 gave any long-range particles. 



The absorption curves are shown in fig. 3. 



* We are very muck indebted for the success of these experiments to 

 the generosity of Dr. H. N. McCoy, who kindly presented one of us with 

 a preparation of radiothorium of activity more than 30 mgr. Ra. Under 

 the best conditions, we were able to obtain sources of ThC on a nickel 

 disk corresponding in activity to 20 mgs. Ea. 



