8i2 Prof. Barkla and Mr. Philpot : Ionization in Gases and 



when traversed by homogeneous beams of X-rays. We 

 give below the results of experiments which have been 

 made at various times within the last four years on the 

 ionizations in a number of gases and vapours produced by 

 homogeneous radiations. 



When a beam of Rontgen radiation traverses a gas, 

 ionization is produced by secondary radiations of three types 

 from the window and walls of the containing-chamber, 

 exposed to the primary radiation, in addition to secondary 

 radiations of three types from the atoms of the gas itself ; 

 consequently the total ionization depends to a large extent 

 upon the particular conditions of the experiment governing 

 the extent to which the gas absorbs these secondary radia- 

 tions. In large ionization-chambers practically all the 

 secondary radiations may be absorbed, while much may 

 escape through the gas contained in small chambers. Without 

 closely reproducing ihe experimental conditions, it is almost 

 impossible from different experiments to get numerical 

 results which show close agreement. It is essential then 

 that in giving the results of investigations it should be 

 precisely stated what the ionization is produced by, that is 

 what radiation is really absorbed in the gas. 



In these experiments homogeneous beams of Rontgen 

 radiation were used as the ionizing agents, and ionization- 

 chambers of various lengths were employed. One of these 

 was a brass cylindrical vessel 12 cm. long and 7 cm. in 

 diameter. This was lined with aluminium in some cases and 

 paper in others. A very thin aluminium wire terminal was 

 placed axially in the cylinder, and was connected through 

 a side tube with an electroscope of the ordinary Wilson 

 pattern. The chamber was provided with inlet and outlet 

 tubes for the admission of gases and vapours. This ionization- 

 vessel and a standardizing ionization-vessel were placed in 

 position so that each received a portion of the fluorescent 

 radiation from a particular element which was subject to 

 the primary X-radiation from an X-ray tube. The usual 

 precautions were taken to screen the ionization-chambers 

 from other radiations. 



The ionization in the ionization-chamber when it con- 

 tained a particular gas was compared with the ionization 

 in the same when it contained air, the radiation being un- 

 changed and the intensity of the radiation being constant, 

 as shown by a constant ionization in a second chamber 

 which contained air. These observations were repeated with 

 many radiators. As the radiation traversing the various 

 gases is absorbed to various extents, the intensity of radiation 



