Question of the Homogeneity of y-Rays. 739 



the weight o£ the absorbing hemispheres it was found 

 necessary to support the platform by four T rods of iron MM 

 running horizontally close beneath the base of the ionization 

 chamber (which consisted of J inch sheet brass), and these 

 probably produced a little secondary radiation capable of 

 penetrating the base of the ionization chamber. Any such 

 secondary radiation would, however, be constant, and would 

 merely increase the value to be subtracted from the readings 

 as " natural leak." 



The results with the lead hemisphere and the y-rays of 

 radium bear out, in a remarkably close manner, equation (6), 

 the value of A, being 0*5. This is practically the value before 

 found, viz., 0495, and is identical with what, as the results 

 of subsequent experiments given in Part III., has been found 

 lead to hold accurately up to a thickness of 20 cm. of lead. The 

 hemisphere was 7*7 cm. diameter. Fig. 4 (PL XII.) shows 

 the results obtained up to the first 2 cm. of lead, and fig. 5 the 

 results for the complete hemispheres with both lead and zinc. 

 Dealing first with lead, the curves drawn are the theoretical 

 curves calculated according to equation (6), with \ = 0*5. 

 Fig. 4 may be first considered, as it is very remarkable. 

 The observational points lie close on the curve, almost from 

 the point at which all the /3-ravs are absorbed. Instead of 

 the large irregularity in the initial part of the curve, as with 

 ordinary dispositions, there is only a very slight departure 

 from the theoretical curve, the slope of the curve initially 

 being a very little greater than the calculated. In fitting 

 the observations to the curves the following procedure was 

 adopted : — By repeated trial with different values of X, that 

 value was found which best fitted the results as a whole. In 

 this way \ was easily determined to about 2 per cent. Then 

 in drawing fig. 4, one observation was put on the theoretical 

 curve somewhere about the middle and the other points 

 plotted from this point as the basis. Dealing now with 

 fig. 5 for the whole lead hemisphere, only the logarithmic 

 curve is shown. Measurements were done with 0*47 and 

 and 6*7 mg. of radium, and showed virtually no difference. 

 To fit the results to the curve and to find the correction for 

 secondary radiation, which at the end of the curve is of some 

 consequence, two observational points, one at the end and 

 the other near the middle, were arbitrarily placed on the 

 theoretical curve (\ = 0'5). A simple calculation then 

 showed what the amount was that had to be subtracted from 

 the observations to allow for secondary radiation, assuming 

 the theoretical law of absorption held true, and this amount 

 was subtracted from all the measurements. It has also been 



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