Radio-Active Matter in ordinary substances. 395 



the matrix the radiation from an infinitely large mass of material 

 will reach a finite limit, and will equal the radiation given out by the 

 mass of radio-active substance within a distance from the observer 

 proportional to the thickness of a layer of the material which 

 would by its absorption reduce the intensity of the radiation to 

 one-half its initial value. Let us call this thickness the coefficient 

 of absorption of the substance. Thus for example if radium 

 or its emanation were uniformly diffused through the air, the 

 radiation coming to the earth from the sky would be proportional 

 to that emitted by the radio-active substance within a hemisphere 

 whose centre is at the place of observation and whose radius is 

 the coefficient of absorption of air, while the amount coming 

 from the ground would be proportional to the radiation emitted 

 by the radium within a hemisphere whose radius is the co- 

 efficient of absorption of the soil. Now if the radium is present 

 as an impurity there is no reason why the quantity per unit 

 volume in air should be to the quantity per unit volume in the 

 ground in the proportion of the coefficient of absorption of air to 

 that of the soil ; so that the effect of screening off the radiation 

 from the sky might be expected to be very different from that 

 produced by screening off the radiation from the ground and also 

 the effect of screening off this radiation might be expected to be 

 very different in different countries. Now Cooke found in his 

 experiments in Montreal that the effect of screening off the 

 radiation from the sky was the same as the effect produced by 

 screening off the radiation from the soil, and experiments made in 

 Cambridge have shown that the reduction in the ionization made 

 by screening in Cambridge is practically the same as in Montreal. 

 Again, the effect of screens made of different materials in 

 cutting off the radiation from outside is not what we should 

 expect if this radiation were one of the forms of radium radiation. 

 The absorption of these has been shown by Strutt to be roughly 

 proportional to the density, so that screens of different materials 

 but of the same mass, should produce approximately the same 

 effect. I have found that this is by no means the case. The 

 experiments were as follows : the vessel in which the ionization 

 was observed was a closed brass cylinder 40 cm. long, 9 cm. in 

 radius and 75 mm. thick, a gold-leaf electroscope observed by a 

 telescope with a micrometer eye-piece fixed inside the cylinder 

 gave the ionization. This cylinder was placed in the middle of 

 a galvanized iron tank which could be filled with water and sand, 

 gravel, sawdust, or bricks. The dimensions of the tank were 

 80 cm. x 60 cm. x 60 cm., so that when the tank was full the vessel 

 containing the air to be tested was, except at the top, surrounded 

 by a screen about 30 cm. thick. Lining the tank with lead, about 

 2 cm. thick, produced a diminution of about 17 per cent, in the 



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