342 Prof. W. H. Bragg on the Ionization of 



Nevertheless, in a number of cases in which I have attempted 

 to calculate the value of HI of one gas from a knowledge of 

 the values of RI for air, and for a mixture in known pro- 

 portions, I have obtained an unexpectedly high result, and 

 when I began some direct experiments on the question I was 

 quite prepared to find that the ionization of a mixture of air 

 and gas was more than the sum of the ionization of air and 

 gas separately. Farther experiment will, no doubt, make 

 everything quite clear. In the meantime it is sufficiently 

 evident that the principle is at least nearly true. For the 

 purpose of this investigation it may be taken as quite true,, 

 since the correction to be made for the presence of air is, at 

 the most, only small. 



The quantity I, as measured, includes a small proportion of 

 ft ray ionization. It must be shown that this does not harm 

 the result. 



In the form of apparatus which I use the ionization in the 

 portion AE of the curve (see fig. 1) is nearly G per cent, of 

 the ionization at P, and I have' not found enough variation 

 from gas to gas to justify an attempt at correcting for it. Of 

 course, the quantity is only small. 



The curve shows only this /3 ionization above A ; that 

 below is hidden. But I have found by experiment that it 

 varies very little throughout the whole distance from the axis 

 of x. This I did by placing over the radium just enough 

 tinfoil to cut off all the a rays. 



In the foregoing will be found, I think, sufficient justi- 

 fication for the choice of the method of this paper, and 

 for the assumptions made during the calculation of the 

 results. 



In the previous paper I have already given a brief descrip- 

 tion of the process of an experiment. Some points, however, 

 deserve reconsideration in the light of further experience, and 

 some changes have been found convenient. These are best 

 discussed in relation to an actual experiment ; I will take a 

 determination of HI in carbon bisulphide. 



I have found it best to separate experiments whose object 

 is to determine III from thoso whose object is to find the 

 stopping power of the gas. In the former the chief difficulty 

 lies in overcoming initial recombination. This requires the 

 pressure of the gas to be low, and the applied potential to be 

 high. A little leakage of air into the apparatus, which can 

 hardly be avoided under these circumstances, is no serious 

 disadvantage, since the proportion of air can be found from a 

 knowledge of R, and of the pressure and temperature at the 



