850 Prof. Barkla and Mr. Philpot : Ionization in Gases and 



Simons * pointed out, if all ionization by X-rays be clue to 

 secondary corpuscular radiation, the difference between tbe 

 ionization in a mixture and the sum of the separate ioniza- 

 tions ought to be well marked. Preliminary experiments 

 did not indicate a difference outside the possible errors of 

 experiment. The differences to be expected from these ex- 

 periments were, however, only small, though apparently 

 outside the limits of experimental error. We have therefore 

 continued these experiments under conditions more favourable 

 to the detection of any difference tbat might exist. This 

 has been done by two different methods. In one the ad- 

 ditional ionizations due to the admixture of a small quantity 

 of vapour of ethyl bromide with two different gases were 

 compared ; in the other the sum of the ionizations in two 

 separate gases was compared with the ionization produced in 

 the mixture of the two gases at the same partial pressures as 

 separately. 



(a) Ionization due to mixture of C 2 H 5 Br with air and 



with SH 2 . 



It has been seen that when a corpuscular radiation is 

 completely absorbed in sulphuretted hydrogen the number 

 of ions produced is about 33 per cent, greater than when the 

 same corpuscular radiation is absorbed in air. If, then, all 

 ionization by X-rays is due to the secondary corpuscular 

 radiation, we are led to expect that when a very small 

 quantity of ethyl bromide say is added to sulphuretted 

 hydrogen, the additional ionization produced is 33 per cent, 

 greater than when it is added to air and ionized under the 

 same conditions, for in each case the corpuscles from the 

 ethyl bromide are absorbed by the gas with which it is 

 mixed. 



When the quantity of ethyl bromide is made so small that 

 the proportion of corpuscles which it absorbs is negligible, 

 the errors of experiment, however, become too large for 

 accurate work. In our experiments we mixed ethyl bromide 

 at pressures of 2, 4, 6, and 10 cm. of mercury with air and 

 with sulphuretted hydrogen, bringing the total pressure in 

 each case to one atmosphere. 



The ionization-chamber was cylindrical in shape, 12 centi- 

 metres long and 7 centimetres in diameter, while the windows 

 and lining were of aluminium. Tli6 ionizing radiation was 

 the Ag X-radiation (series K). The results given in 

 Table VI. are the average of several determinations with 

 each quantity of ethyl bromide. 



* Phil. Mag. Feb. 1912, pp. 317-333. 



