406 Prof. W. H. Bragg on the Consequences of 



a deficiency from the proportionality to pressure, as already 

 said, and hence bis conclusion. 



But it is clear that this experiment proves too much. One 

 o£ the gases he used was methyl iodide. If X rays strike 

 iodine atoms there is a very large conversion into cathode 

 rays, as may be shown easily by scattering a little iodoform 

 on a card through which X rays are entering an ionization- 

 chamber when the current may be doubled under quite usual 

 conditions. When the thinnest sheet of tissue-paper, equi- 

 valent to 1*5 cm. of air, is laid over the iodoform, this extra 

 radiation is absorbed and the current returns to its former 

 value. It is a clearly established principle that the effect of 

 an atom upon an X ray is the same, no matter whether the 

 atom is part of a solid or of a gas. Consequently there is a 

 large production of cathode rays in the vapour of methyl 

 iodide through which X rays are passing*, and a considerable 

 fraction of the ionization of the gas is caused by these cathode 

 rays. The amount can be calculated on the principles laid 

 down above ; but even if the complete accuracy of such a 

 calculation be denied, it is still clear that the cathode ray 

 ionization is large. Yet Crowther found there was none 

 at all. Again, Mr. Edmonds has shown in this laboratory that 

 if a hole is made in one of the parallel plates of Crowther's 

 experiment and a piece of wire gauze placed over it, cathode 

 rays pass through the hole from the X-ray stream in quantities 

 which show a large increase as soon as the pressure of the 

 air is sufficiently lowered. The distance from the X-ray 

 stream to the window is about a centimetre, and the ionization 

 current which is measured on the side of the gauze away 

 from the stream increases rapidly relatively to the ionization 

 in the air through which the X rays are passing : at first 

 there is even an absolute increase in spite of the lowering 

 of the pressure. The relation of the increase to the pressure 

 alterations is just such as would be expected if the ionization 

 outside the gauze window was due to cathode rays made in 

 the X-ray stream and passing through the meshes in the 

 gauze. 



If the ionization of the gas in an ionization-chamber across 

 which y rays are passing is caused wholly by the /3 rays 

 coming out of the walls of the vessel or out of the atoms of 

 the gas, then, since the former of these sources of ft rays 

 is usually far more important than the latter, the ionization is 



* It is worth observing that in a mixture of methyl iodide and any gas 

 of small atomic weight the iodine atoms would be responsible for a large 

 ionization, but only a fraction of the ions would be formed from the methyl 

 iodide molecules. 



