726 Prof. Rutherford and Mr. Richardson : Analysis 



the sides, the vapour was introduced by means of a slow 

 current of hydrogen which bubbled through the liquid. 

 Under these conditions, the ionization vessel was filled with 

 a mixture of hydrogen and vapour of methyl iodide at atmo- 

 spheric pressure. The ionization in the vessel was almost 

 entirely due to the methyl iodide, and for the hard y rays 

 from radium C was usually about three times as great as for 

 air at atmospheric pressure. Some difficulty was at first 

 experienced on account of the absorption of the vapour by 

 the wax used in fixing the mica plates and in sealing the 



Fie. 2. 



2 2-5 3 



CMS. Of ALUMINIUM. 



various parts of the vessel. This effect was got rid of by 

 reducing the amount of wax to a minimum, and covering it 

 with a non-absorbing layer of gum. This was kindly pre- 

 pared for us by Dr. Lapworth, F.R.S., by special treatment 

 of the ordinary gum sold commercially. By this method 

 the absorption of the vapour was so much reduced that 

 measurements extending over several hours could be made 

 with certainty and accuracy. 



The absorption curve using methyl iodide instead of air 

 is shown in fio\ 2 B. An absorbing screen of l'G mm. of 

 aluminium reduced the ionization to about 50 per cent. It 

 will also be seen that the latter part of the absorption curves 

 A and B are not linear and are not parallel to one another. 



