the Cause and Nature of Radioactivity. 



571 



II. Method of Measuring Emanating Power. 



The emanation from thorium (and from radium) behaves 

 in all respects like a temporarily radioactive gas, and diffuses 

 rapidly through porous substances, as, for example, thick 

 cardboard, which are completely opaque to the straight line 

 radiation. Each particle of the emanation behaves as if it 

 were a radiating centre, producing charged carriers through- 

 out the gas in its neighbourhood. The emanation passes 

 through plugs of cotton-wool and can be bubbled through 

 liquids without appreciable loss of radioactivity, whereas the 

 charged carriers, produced by the emanation in common with 

 the straight line radiation from radioactive substances, on the 

 contrary, completely disappear on passing through a plug of 

 cotton- or glass-wool, or by bubbling through liquids. The 

 means of eliminating the effects of the straight line radiation 

 and of measuring the amount of the emanation alone thus 

 suggest themselves. Air passed over uranium or other non- 

 emanating radioactive substance will no longer conduct a 

 current after passage through cotton-wool. The conductivity 

 in the case of thorium, however, will persist, and afford a 

 measure of the amount of emanation present. 



Fig. 1 shows the experimental arrangement for comparing 



FiR-. 1. 



CASOM£TEP, 



the emanating power of substances. These are placed in the 

 form of fine powder in a shallow lead vessel inside the glass 

 cylinder, C, 17 cm. in length and 3*25 cm. in diameter, pro- 

 vided with indiarubber corks. A current of air from a large 

 gas-bag, after passing through a tube containing cotton-wool 

 to remove dust particles, bubbled through sulphuric acid in 

 the vessel A. It then passed through a bulb containino- 

 tightly packed cotton-wool to prevent any spray being carried 

 over. The emanation mixed with air was carried from the 

 vessel C through a plug of cotton-wool, D, which completely 

 removed all the charged carriers carried with the emanation. 

 The latter then passed into a long brass cylinder 75 cm. in 



2 P2 



