164 Mr Cooke, Experiments on Penetrating Radiation. 



pond, the ionization increased between 5 and 10°/o- Suspending 

 the apparatus by a wire over the surface of the pond gave results 

 similar to when the vessels were exposed on the neighbouring 

 lawn, or on the roof of the laboratory. This increased radiation 

 may be due to the banks of the pond, and the presence of a large 

 number of trees in the immediate neighbourhood. 



The theory has been suggested that all substances may give rise 

 to a penetrating radiation, and that the ratio of the amount of 

 this activity to the density of the substance may vary in different 

 materials. When an ionization vessel is surrounded on all sides 

 by a thickness of any substance sufficient to absorb all external 

 radiation, the ionization in the enclosed gas assumes a definite 

 value, which depends on the ratio of activity to density of the 

 shielding material, and which is unaffected by additional layers of 

 the screening substance. Under such circumstances the greatest 

 reduction in ionization observed has been 35°/o of the value 

 obtained when the apparatus was unscreened indoors. How much 

 of the remaining 65°/o is due to a penetrating radiation from the 

 screening material, and how much is due to a radiation of a more 

 easily absorbed type from the walls of the vessel, no experiments 

 up to the present have indicated. The results so far show that 

 in water, lead, iron, and possibly the atmosphere, this ratio of 

 activity to density is small, but there is no means of estimating 

 how near zero this ratio is. The results are merely comparative. 

 The experiments up to the present with the apparatus buried 

 have not been sufficiently extensive to draw accurate conclusions 

 from, but there are indications that for earth this ratio is higher 

 than for water, iron, and lead. 



The results obtained up to the present may be summarized as 

 follows. The penetrating radiation inside buildings is about 25°/ 

 greater than that outside on the roof. The ionization with the 

 vessel on a lawn and suspended over the surface of a tank is ap- 

 proximately the same as on the roof. Submerging the apparatus 

 to a considerable depth in a spring-water tank further reduces the 

 leak by about 12°/ , the reduction not being so great with the 

 apparatus floating on the surface. The ionization with the vessels 

 buried about a foot in the ground does not differ greatly from the 

 results on the roof. 



The work is being continued. The author wishes to express 

 his sincere appreciation of Professor Thomson's unfailing interest 

 and many useful suggestions during the course of the work. 



