434 Dr. J. P. V. Madsen on 



observers. To obtain with great accuracy the zero reading- 

 necessary for subtraction, is a course exceedingly difficult. 



In the present experiment a close approximation to its 

 value was found by inserting a large Pb plug in the conical 

 hole and placing plates of Pb above this, so as to make in 

 all a thickness of about 20 cm. This, as shown by previous- 

 observers, should be sufficient to reduce even the hard y 

 radiation, which streamed through the conical hole, to almost 

 a negligible amount, while it should not affect to any great 

 extent the secondary radiation coming from surrounding 

 bodies. 



The results of experiments with different materials used 

 as .radiators are given in Table II., and in fig. 8 are shown 

 by full-line curves the thickness of radiator multiplied by 

 its density, plotted horizontally against the logarithm of the 

 corresponding currents vertically. The value of the current 

 with no radiation in position at|>, p has in each case been 

 reduced to the common value 100, and the zero for the 

 ordinates has in some of the curves been altered so as to 

 prevent overlapping. It will be seen that Pb, Bi, and Hg 

 give results almost identical ; while Sn, Zn, Al, and C all 

 give results very similar to each other, but differing from 

 those of the first group in that a more sudden drop of the 

 curve is shown for the initial thicknesses of radiator. Prom 

 a value of dA = 7 onwards the curves for all the substances 

 become practically parallel. 



The Pb screens L, L, fig. 7, w~ere now 7 removed, and 

 radiators were used in the form of large, flat plates, the 

 first of which was placed horizontally just below the floor 

 of the ionization-chamber, merely separated from it by a 

 small thickness of ebonite for insulation. 



Successive thicknesses of radiator were added immediately 

 below the first. By this means it was thought possible to 

 allow a large proportion of all the emergent secondary y 

 radiation to add on its effect in the chamber. The results 

 of these experiments, given in Table III. and shown by the 

 dotted curves in fig. 8 (PL VIII.) show that this anticipation 

 was realized. /— ^ ^— x 



The dotted curves ( AY J and ( z n Mneed not be considered 



at present ; they were obtained with a carbon floor intro- 

 duced into the ionization-chamber, and show the effects which 

 previous work w r ould lead us to expect. 



Al, C, Zn, Fe, S, and Sn give results which are all very 

 nearly identical, and the curves show a considerable difference 

 in slope to what they did in the previous experiment. 



