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[ 46 ] 



IV. Delta Rays. — II. By Norman Campbell *. 



1. TITHE following pages contain an account of experiments 

 _1_ performed in continuation of those recently described 

 under the same title f. 



At the end of the former paper the question was discussed 

 at some length whether the differences observed when the 

 delta rays were excited in different metals were due to 

 differences in the speed of the rays or to a difference in the 

 reflecting powers of the metals for the rays. No certain 

 decision was attained, though the evidence was on the whole 

 in favour of the view that a difference in the speed had not 

 been demonstrated. It was thought that the matter might 

 be definitely settled, if in place of the metals soot were 

 employed as one of the electrodes, since Baeyer % has shown 

 that this substance has practically no reflecting power. 

 Hitherto attempts to use soot had been unsuccessful owing 

 to uncertainties in the resulting measurements. These diffi- 

 culties have now been overcome by certain slight changes 

 in the apparatus and in the method of covering the electrodes 

 with soot ; in all important characteristics the apparatus is 

 the same as in the experiments described previously. In the 

 later observations an electrometer was used in place of the 

 electroscope as the indicating instrument of the measuring 

 system. In so far as there is conflict between the present 

 experiments and those already described, I am inclined to 

 attribute greater weight to the more recent work. 



2. The measurements which had been made previously 

 with the electrodes covered with aluminium, copper, silver, 

 and gold were repeated and similar measurements made with 

 the electrodes covered with soot. The results are given in 

 Table I. and fig. 1. The significance of the numbers in the 

 columns marked +, — , and "sum" is the same as that of 

 the numbers in the corresponding columns of Tables 3-5 of 

 the previous paper. In fig. 1 (p. 48), which is similar to fig. 3 

 of the previous paper, the results of the " sum iy columns 

 are plotted. 



A comparison with the previous work will show that the 

 general trend of the curves for aluminium, copper, silver, 

 and gold and the general relations between the curves is the 

 same as in the previous work. But the results are not com- 

 pletely identical ; it had been found before that very slight 



* Communicated bv the Author. 



t Phil. Mao-. Aug;. 1911. p. 276. 



% Baeyer, Fhys. Zeit. x. p. 176 (1909). 



