438 Sir J. J. Thomson on the Origin of 



it excited a stream of electrons a million times greater than 

 that which could have been detected if any had emerged from 

 the vapour. Ultra-violet light of very small wave-length 

 does undoubtedly produce ionization in gases, while one of 

 the most characteristic properties of the Rontgen rays is their 

 power of ionizing gases. We must remember, however, 

 that as the absorption-bands in the ultra-violet of gases like 

 C0 2 and 2 show, gases may in many regions of the ultra- 

 violet light spectrum be able to give out a spectrum which 

 is continuous over many frequencies, and presumably, there- 

 fore, possesses electrons able to vibrate freely over a wide 

 range of: frequencies. Again, it is important to remember 

 that the number of atoms ionized is an exceedingly small 

 fraction of the number of atoms in the gas. The number 

 varies, of course, with the intensity of the ionizing radiation ; 

 but under ordinary conditions of laboratory work the number 

 of atoms ionized per second is much less than 1/10 12 of the 

 number of atoms exposed to the radiation ; or, to put it 

 another way, an individual atom would not be ionized nearly 

 so often as once in one hundred thousand years. 



Now, in a gas the atoms come into collision and may form 

 temporary associations, and while in this state the natural 

 frequencies are changed by an amount which will vary from 

 one pair of atoms or molecules to another; hence, if we take 

 these pairs into account we shall find in the gas, electrons 

 whose natural frequencies are not merely those of the free 

 atom, but extend continuously over the range of frequencies 

 to be found in the pairs. Thus, as far as range of frequencies 

 of the electron goes, the gas will be on an equality with a 

 piece of metal. It is true that in the gas the number of 

 electrons which can vibrate in what we may call these 

 abnormal frequencies is an exceedingly small fraction of the 

 whole number of electrons ; but we have seen that if the 

 electron gets into this state once in a hundred thousand years 

 it will be able to do more than its full share of the ionization 

 in the gas. The effect of proximity of the atoms in increasing 

 the range of frequencies is shown by the enormous broadening 

 of some lines when the pressure of the gas is increased.. 

 If we accept the view that in a gas we may expect to find 

 electrons with frequencies varying continuously over a wide 

 range, though the number which have frequencies differing 

 from the normal natural frequencies may be exceedingly 

 small, we may apply to gases the reasoning that we use for 

 the photoelectric effect in metals, and assume, as the result, 

 that the energy in the ejected electron will be proportional 

 to the frequency of the radiation which supplied it with 



