544 Radiation from Ordinary Materials, 



§ 14. No evidence whatever could be obtained o£ the 

 existence of rays from ordinary materials more penetrating 

 than those which have hitherto been considered. If the window 

 of the box was covered with a layer of aluminium '003 cm. 

 thick, no increase amounting to 2 per cent, of the normal leak 

 was caused by placing before the window any of the ordinary 

 materials used in these experiments. Such a thickness of 

 aluminium would scarcely cause an appreciable diminution 

 in the intensity of rays as penetrating as the /3 rays from 

 radium or uranium. Hence, again we are led to the con- 

 clusion that v 1 of § 6 is zero. 



§ 15. The conclusions which are considered to be established 

 by the experiment described in this paper may be put briefly 

 as follows : — 



(1) That the influence, which the walls of a containing 

 vessel are known to exert upon the spontaneous ionization of 

 the enclosed air, may be attributed to a radiation proceeding 

 from the walls. 



(2) That part of this radiation from certain materials, such 

 as tin, zinc, graphite, and platinum, is analogous to the 

 secondary radiation excited by Rontgen and other rays, being- 

 caused by the penetrating ionizing rays which Cooke has cut 

 off with thick lead screens. 



(3) That the absorption coefficient of air for this radiation 

 is comparable with that of air for the a. rays from radium. 



(4) That the absorption coefficient is different for different 

 materials, and hence it is unlikely that the radiation is due to 

 radioactive impurities ; it is more probably an inherent 

 property of the material *. 



(5) That there is no evidence of the existence of rays from 

 ordinary materials of a penetrating power considerably greater 

 than that of the a rays from radioactive elements. 



It is obviously a matter of great importance to determine 

 the nature of the rays from ordinary materials — their charge, 

 if they carry a charge, their velocity, and so on. But the 

 investigation of these properties will tax the ingenuity of the 

 ablest experimenter. 



These experiments were carried out in the Cavendish 

 Laboratory, Cambridge. When this is stated it is hardly 

 necessary to add that a large portion of whatever success has 

 attended them is due to the inspiring advice of Prof. Thomson. 



Cambridge, Jan. 1905. 



* Most of the radioactive elements give off an emanation. It is here 

 suggested that all elements are radioactive to some degree. It might 

 be expected that some would give off an emanation. All efforts to 

 directly detect such an emanation from lead, zinc, and aluminium have 

 hitherto failed. 



