584 Sir E. Rutherford on Collision of 



at N.T.P. is introduced in place of carbon dioxide. The 

 difference curve C shows the corresponding variation of the 

 number of scintillations arising from the nitrogen in the air. 

 It was generally observed that the ratio of the nitrogen effect 

 to the natural effect was somewhat greater for 19 cm. than for 

 12 cm. absorption. 



In order to estimate the magnitude of the effect, the space 

 between the source and screen was tilled with carbon dioxide 

 at diminished pressure and a known pressure of hydrogen was 

 added. The pressure of the carbon dioxide and of hydrogen 

 were adjusted so that the total absorption of a particles in the 

 mixed gas should be equal to that of the air. In this way it 

 was found that the curve of absorption of H atoms produced 

 under these conditions was somewhat steeper than curve C of 

 fig. 1. As a consequence, the amount of hydrogen mixed 

 with carbon dioxide required to produce a number of scintil- 

 lations equal to that of air, increased with the increase of 

 absorption. For example, the effect in air was equal to 

 about 4 cm. of hydrogen at 12 cm. absorption, and about 

 8 cm. at 19 cm. absorption. For a mean value of the 

 absorption, the effect was equal to about 6 cm. of hydrogen. 

 This increased absorption of H atoms under similar conditions 

 indicated either that (1) the swift atoms from air had a some- 

 what greater range than the H atoms, or(2) that the atoms from 

 air were projected more in the line of flight of the a particles. 



While the maximum range of the scintillations from air 

 using radium C as a source of a rays appeared to be about the 

 same, viz. 28 cm., as for H atoms produced from hydrogen, 

 it was difficult to fix the end of the range with certainty on 

 account of the smallness of the number and the weakness of 

 the scintillations. Some special experiments were made to 

 test whether, under favourable conditions, any scintillations 

 due to nitrogen could be observed beyond 28 cm. of air 

 absorption. For this purpose a strong source (about 60 mg. 

 Ra activity) was brought within 2'5 cm. of the zinc sulphide 

 screen, the space between containing dry air. On still further 

 reducing the distance, the screen became too bright to detect 

 very feeble scintillations. No certain evidence of scintillations 

 was found beyond a range of 28 cm. It would therefore 

 appear that (2) above is the more probable explanation. 



In a previous paper (III.) we have seen that the number 

 of swift atoms of nitrogen or oxygen produced per unit path 

 bv collision with ex. particles is about the same as the corre- 

 sponding number of II atoms in hydrogen. Since the number 

 of long-range scintillations in air is equivalent to that produced 

 under similar conditions in a column of hydrogen at 6 cm. 



