850 Ionization of Gases by the a. Particles of Itadiuui. 



in numbers which may have a bearing on this question. 

 Ramsay and Soddy (Proc. Roy. Soc. lxxii. p. 204, 1903) found 

 that 50 mmg. of radium bromide in solution evolved gases 

 at the rate of '5 cc. per day — i. <?., 2 x 10 19 molecules per day. 

 Now, Rutherford has shown that one gram of radium bro- 

 mide, without its radioactive descendants, produces 3*6 x 10 la 

 a. particles per second. Each a. particle makes 86,000 pairs 

 of ions. Hence the number of ions made in one day by 

 50 mmg. is 



3-6 x 10 10 x -05 x 60 x 60 x 24 x 172,000 = 2*7 x 10 19 . 



This number is an inferior limit. A superior limit is 

 found by considering all the radioactive products of radium 

 to be present in full, in which case the number will be 

 between five and six times greater. The close agreement of 

 these numbers certainly fits in with the hypothesis that an 

 actual disruption of the water molecule takes place in con- 

 sequence of the passage of the a particle through it. 



I owe my thanks to my assistant, Mr. A. L. Rogers, for 

 the great care and skill with which he has made the apparatus 

 used in this work, and drawn the plate illustrating this paper. 



Nov. 14. — The very interesting experiments described by 

 Kucera and Masek in recent numbers of thePhys. Zeit. afford 

 a very welcome confirmation of the square-root law of 

 absorption . 



On one point only there is still some obscurity. It was 

 pointed out by Kleeman and myself (Phil. Mag. Sept. 1905) 

 that the a particles from Ra A were less stopped than those 

 from Ra C by a sheet of metal placed at a given distance from 

 the Ra. (The distance was 1*5 cm. in our experiments, not 

 zero as Kucera and Masek suppose ; the point is of some 

 importance in view of their comparison of our results with 

 their own.) This could be explained by supposing that " the 

 slower a particles were a little less affected than the swifter '* 

 (Phil. Mag. Sept. 1905, p. 337). Kucera and Masek come 

 to the conclusion that they have established this fact from 

 their own experiments. The trouble is that this is hardly 

 consistent with Rutherford's experimental determination that 

 successive equal layers of Al foil absorb equal energies 

 from the ct particle's motion. Unless indeed it is supposed 

 that metals absorb to an extent which is independent of speed, 

 and that it is the air which is at fault; for the experiments of 

 Kucera and Masek and those of Kleeman and myself relate 



