Rbntgen Radiation from Gases and Vapours. 6G3 



hydrogen, nitrous oxide, and so on, it was quite imperceptible ; 

 while in no case did it amount to more than 10 per cent. ; 

 the greater absorptive powers of the heavier vapours being 

 counterbalanced by the very small pressures at which they 

 were measured. 



We have been assuming, up to the present, that the 

 characters of the secondary rays, given off by the various 

 gases, under the action of the same incident beam, are 

 identical. If this is not the case, we shall have still further 

 corrections to make. In many cases, as will appear later, 

 the assumption was correct ; the secondary rays being 

 always identical in penetrating power with the primary rays 

 producing them. Some gases and vapours, however, were 

 found to give off secondary rays of a much more absorbable 

 type than those given off under the same conditions by gases 

 of the first class. To these gases, therefore, further cor- 

 rections must be applied : — 



(i) For the greater absorption of the emitted rays by the 

 aluminium window d. 



(ii) For the alteration in the amount of ionization in the 

 ionization-ch amber due to the altered character of the 

 rays producing it. 



The first correction can be readily applied. It is easy to 

 measure the amount by which the aluminium window cuts 

 down the secondary radiation from air, and knowing this 

 and the coefficient of absorption of the secondary rays from 

 any other gas, to calculate the amount by which the radiation 

 from the latter is reduced. The actual amounts will, of 

 course, vary with the hardness of the primary rays. The 

 measurements recorded were made when the bulb had an 

 equivalent spark-gap of between 4 cms. and 4-J- cms. With 

 this hardness the aluminium window d cut down the radiation 

 from air by about 10 per cent. 



It is much less easy to allow for the second effect. The 

 intensity of the secondary radiation is measured by the 

 relative amount of ionization produced in the ionization- 

 chambers, and it is assumed that the ratio of the two ioni- 

 zation- currents is an accurate measure of the ratio of the 

 amounts of energy radiated upwards from the gas-chambers. 

 As long as the radiation from the two chambers is of the 

 same character this assumption is, no doubt, quite justifiable, 

 but it is almost certainly incorrect when the rays differ in 

 hardness. I do not know of any experimental evidence as 

 to how the ionization in a given gas, say air for example, 

 varies with the hardness of the rays producing it. The only 

 methods at present available for the measurement of X-rays 



