the Corpuscular Hypothesis of the 7 and X Rays. 403 



appreciable quantities. At ordinary pressures the formula 

 becomes I(DA-' + kj\). 



In this form it may be tested experimentally. It may be 

 well to repeat that this formula is deduced on the suppo- 

 sition that X rays do not ionize a gas directly, but indi- 

 rectly through the intermediate action of the cathode rays 

 produced by the X rays in the metal through which they 

 enter and in the gas which they cross. The term lk/\ 

 represents the effect due to the cathode rays from the metal ; 

 ID/o' represents the effect due to the cathode rays formed in 

 the gas. The first of these can be determined by experiment 

 in a given case ; the second can be calculated from the first 

 when measurements have been made of k/k', \, and D. If 

 then the ionization produced by the X rays in the gas 

 (directly or indirectly) is also found experimentally, it can 

 be seen whether the calculated indirect effect is sufficient to 

 account for it all, or whether there is something left over which 

 must be ascribed to the direct action of the X rays. 



I have made a number of experiments of this kind and 

 have found that the results were always to be explained on 

 the supposition that there was no direct action of the X rays. 

 An example will show the usual extent of the agreement. 



An ionization-chamber was made of brass, lined with 

 aluminium to avoid disturbances due to the secondary X rays 

 of brass, and again with paper to cut out the secondary cathode 

 rays from the aluminium. The chamber was cylindrical, 

 3*6 cm. deep and 10 cm. in diameter. A pencil of primary 

 X rays was passed in along the axis through an opening 

 1 cm. in diameter. When a card was placed over the 

 opening, and nine thicknesses of silver-foil placed on the 

 card on the side next the ionization-chamber, the current 

 was 150*0 on an arbitrary scale : when the foils were placed 

 the other side of the card the current was 70*3. The difference 

 79*7 was due to the cathode rays from the silver : i. e. we 

 may take lk/\ to be 79*7. The absorption coefficient k was 

 then found by placing various thicknesses of silver under the 

 card, and measuring- the current in each case. The curve 

 obtained when the results were plotted was not far from 

 exponential, and gave k equal to 43'2 for the primary rays 

 after passing through 9 foils. The absorption coefficient 

 required is that which measures the conversion into cathode 

 ray energy, excluding secondary X rays. It is therefore 

 better to put the absorbing sheet close to the ionization- 

 chamber so that secondary X rays may be taken in, though 

 there is still some error due to the difference in quality of the 

 primary and secondary rays. The absorption coefficient for 



' 2 E 2 



