210 Mr. F. H. Parker on some 



destroyed. This procedure gives an exponential law of 

 absorption for homogeneous rays ; but in experiments this 

 law should certainly be regarded not as the result of homo- 

 geneity but of extreme heterogeneity and of scattering. 



If/; is the greatest distance from the nucleus for which a 

 J3 particle is caught and if N is the number of: atoms in a c.c, 

 the mean free path of a particle is l/N^p 2 , and the absorption 

 coefficient A, is l^irp 2 . For lead X = 4x 10 22 , and for rays of 

 velocity *9 c (that is fairly hard /3 rays), jt? cr it. = l"35 X 10" 11 . 

 This gives \=20cm. -1 . If the effect of radiation is to be 

 included, p should be taken about three times as great, and 

 this multiplies \ by about 10. This calculation neglects all 

 scattering by the nuclei of those particles which are not 

 absorbed, and also the effect of the electrons in the atoms. 

 These should still further increase the coefficient. For rays 

 of this type the experimental value is somewhere between 

 100 cm. -1 and 500 cm. -1 . 



When caught the /3 particle emits a stream of radiation. 

 This may be supposed to be the secondary X ray. Our 

 calculation suggests that this will be on the scale of 10 ~ n cm., 

 a value very much smaller than is indicated by other 

 methods. 



My thanks are due to Sir J. J. Thomson and to Professor 

 Rutherford for the interest they have taken in this work and 

 for their advice. 



Dec. 4, 1912. 



XXVI. Some Properties of Red Vulcanized Fibre. 

 By F. H. Parker, B.Sc, Woolwich Polytechnic *. 



THE attention of the author was drawn to the fact that 

 vulcanized fibre is one of the few materials, if not the 

 only material suitable for the pistons of compressing engines 

 in the plant for the liquefaction of air, and as, in order to 

 prevent jamming, allowance has to be made for the expan- 

 sion of the fibre, it seemed desirable to make some measure- 

 ments on the change of volume due to absorption of water, 

 as no data appeared to be available. 



The results were rather of an unexpected character and 

 seem of some interest. 



The specimens examined were cut from two sheets, 1*9 cm. 

 and 1*35 cm. thick respectively. 



* Communicated by the Author. 





