230 PHYSICAL SCIENCE 



illumination, but a number of bright specks 

 scattered over a dark ground. Each tube of 

 force would convey its own tremors, and these 

 would constitute light, but between them would 

 lie undisturbed seas of aether. 



Such an idea about the nature of a wave-front 

 of light is very unexpected and surprising. We 

 are inclined at once to relegate our tubes of force 

 to a museum of conceptual curiosities. But it is 

 a remarkable thing that certain evidence in favour 

 of the discontinuous nature of a wave-front of 

 light really does exist. It is impossible to examine 

 the luminous effects with enough magnification to 

 investigate the question, but, as we have seen, 

 ultra-violet light, and still more effectively Rontgen 

 rays, are capable of ionizing a gas through which 

 they pass. Here, it is the molecules of the gas 

 which are affected, and, in examining the ionizing 

 power of the rays, we are in effect using on them 

 a microscope of molecular dimensions. 



If the wave-front of a Rontgen pulse were 

 continuous, all the molecules of the gas would be 

 subject to the same disturbance. But, even with 

 the strongest ionizing agency, nothing like one 

 molecule in a million is found to be affected. 

 Thus, if the wave-front be continuous, we must 

 suppose that it is only those very few molecules 

 which are in some peculiarly receptive state that 

 are ionized. The stability of a molecule is 

 greatly affected by temperature, and, if a critical 

 limit of stability were needed for a molecule to 

 become ionized by the rays, we should expect 

 that the ionizing power would increase rapidly 

 with the temperature. Mr M 'Clung has shown, 

 however, that temperature has no appreciable 



