Collisions of a Particles with Hydrogen Nuclei. 931 



number of H particles observed on the screen per minute 

 per mg. activity of the source for each diaphragm : — 



Diaphragm. ^o. of H particles F(0) _ w Q 



r D per mg. per mm. ^ 2J y 7J 



A 1-7 -29xl0- 5 ll°-3 -39xlO- 5 



-B 7-1 l-23xlQ- 3 21°-4 l-63xl0~ 3 



5-5 -99X10- 3 31°-3 2-94xl0~ 5 



D 4-0 -76xlO- 5 41°-0 3-94X10" 5 



E 2-4 -48X10-* 48 o. 4 4 .5 8xl0 - 5 



The third column gives the values of F(0 2 ) — F(#i) cal- 

 culated according to the final result of § 3, putting 



Q = No. of a particles emitted by 1 mg. Ra per second 

 = 3-7 Xl0 7 



7= 1-22 



and substituting for r 2 the values pertaining to each 

 diaphragm . 



The fifth column gives the numbers of H particles pro- 

 jected within various angles, as calculated from the data of 

 column 3. These are also given in curve B of fig. 4. In 

 this calculation, a correction is necessary for the fact that 

 all the H particles which fall within the field of view of the 

 microscope do not hit a ZnS crystal, and therefore do not 

 produce a scintillation. The efficiency of the screen was 

 determined by comparison with a standard screen, which 

 had been carefully calibrated by sources of RaC, and found 

 to be 76 per cent. 



The values of n are probably correct to within 5 per cent. 



In addition to the numbers of column 5 above, one point 

 is plotted on curve B of fig. 4, corresponding to an angle 

 of 66°. Owing to the fact that, at large angles, the u par- 

 ticles scattered from the C atoms in paraffin wax have a 

 range almost equal to that of the projected H particles, 

 it was found impossible to use paraffin wax in the deter- 

 mination of this point. Hydrogen gas was used instead, 

 and two diaphragms of angular limits 48°*4 and 66° were 

 so arranged that the second one cut out any a particles 

 scattered in the direction of the screen by the first one. 



The number of H particles projected at various angles by 

 a particles of mean ranges S'2, 4*3, and 2'9 cm. was deter- 

 mined in a similar way. 



The source of a particles of mean range 8*2 cm. u as 

 a deposit of thorium 0, obtained on one side of a nickel 



