by a Particles from Radium Emanation. 909 



for absorbing lead bases of thickness 18'5 mm., 12 mm.? 

 5 mm., and about 2 mm. respectively. It will be noted that 

 the curves do not start from zero, but from points the lowest 

 of which is above O'Ol per cent., which Moseley and 



Fig. 2. — Rise curves witli lead target. 



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Makower give as the upper limit of contamination with such 

 a method, However, even when the target bombarded is 

 india paper or the glass of the containing walls, the curve 

 does not start from zero as indicated by theory, and with 

 the thinnest absorbing bases begins higher than O'Ol per 

 cent. It should be pointed out, however, that Moseley and 

 Makower (loc. cit.) were not interested in the first part of 

 the curve, and probably concluded, by extrapolation, that 

 the curve would always go back to within 0*01 per cent, 

 of the maximum. This extra initial activity over that 

 indicated by theory, obtained when paper lined the tube, 

 may be due to contamination, or to radiation excited in the 

 paper by the a particles, or to radiation excited in the active 

 material itself, i. e., the emanation atoms. The curves 

 when paper lined the tube, by comparison with the lead 

 curves, show a marked difference between the amount of 

 radiation given by the two substances at zero time. The 



