622 -Photographic Registration of a Particles. 



The line B corresponds to a movement of the film of 180 cm. 

 per second and for about 600 a particles per minute. The 

 line C corresponds to an average of 900 a particles per 

 minute. 



It is seen that a clear record is obtained of the movements 

 of the fibre even when on an average 15 a. particles enter per 

 second. With eye observations, the movements of the fibre 

 appeared blurred and confused, but the photograph brings 

 out clearly each individual movement. With a little ex- 

 perience, it is not difficult to count the number of a particles 

 that have entered the detecting vessel in a given interval 

 even at the rate of 1000 per minute. 



The experiments described in this paper have been made 

 with a view to determinino- the best conditions for measuring 

 with accuracy the number of a particles expelled from radio- 

 active substances, and in particular, for re-determining the 

 number of a. particles expelled per second from one gram of 

 radium. A lengthy series of observations will be required 

 before the value of this important constant can be definitely 

 settled. 



Recoil atoms. — It is well known from the experiments 

 of Hahn * and of Russ and Makower f, that the emission of 

 a particles from radioactive substances is accompanied by a 

 vigorous recoil of the residual atoms. The recoiling atom is 

 able to penetrate about 1/10 of a millimetre of air and of 

 about 4/10 of a millimetre of hydrogen at atmospheric 

 pressure, before being stopped. Wertenstein J has shown 

 that these recoil atoms produce a strong ionization in the 

 gas they traverse. This has been observed by determining 

 the ionization due to recoil atoms and a particles at low 

 pressures. Under these conditions, the recoil atom appears 

 to cause several times the ionization due to an a particle over 

 the same range. 



We have seen above that it is not difficult to detect the 

 electrical effect of a single a particle when it traverses a 

 gas at such a low pressure that its range is only reduced by 

 1/20 of a millimetre. This is less than the range of the 

 recoil atom, and it thus seems probable that the electric 

 method could be employed to detect a single recoil atom. 

 Some experiments were made as follows : — the mica w T indow 

 over the opening of the detecting vessel was removed and 

 the radiation from a plate coated with the active deposit of 

 actinium was allowed to enter the detecting vessel, which 



* Hahn, Verh, d. D. phys. Ges. xi. p. 55 (1909;. 



t Russ & Makower, Proc. Roy. Soc. A. lxxxii. p. 205 (1909). 



X Wertenstein, U. -R. clii. p. 1657 (1911). 



