JoLY — Radio-Tlicrapji : its Scientifie Basis and its Teaching. 497 



If now an electric potential is established — say, between two electrified plates 

 contained in the gas — the pins electrons move towards the negative plate and 

 the negative electrons towards the positive plate. Finally, tiie plates become 

 in this manner neutralized or discharged. The ions, in fact, confer the 

 property of conductivity upon the gas. 



Ions can exist in liquids, and can be moved therein by electric forces, as 

 in the case of a gas. The slow motion of the ions in an electrolyte confers on 

 it its conductivity. In recalling these trite facts, we would emphasize tlie 

 ubiquity of the ion. We might advance yet further. The conductivity of 

 solids is very probably due to the free motion of electrons within them. It is 

 probable that these electrons, the mobility of which is peculiar to conductors, 

 stand in a different relation to the molecules of the substance from electrons 

 concerned with the chemical forces attracting atom to atom. The latter 

 class of electron we shall now consider. 



The chemist to-day recognizes that ionization is at the basis of chemical 

 activity, the force of attraction between the positive and the negative ion 

 playing a fundamental part. As we have seen, the loss of an electron 

 converts the neutral atom to the positive ion ; the addition of one converts 

 the neutral atom to the negative ion. 



It is through the agency of the ion that tlie light ray, or tlie j ray, effects 

 chemical changes. The freeing of the electron is the primary effect. Such 

 electrons — i.e. those concerned with the chemical bonding — are termed 

 valency electrons. How important tliis action is in Nature will be realized 

 when we consider that the metabolism of green plants the world over depends 

 on the photo-electric activity of the sun's rays. 



We might fill much space in merely listing the cases of photo-chemical 

 changes which have been studied within recent years. The chemical changes 

 seem in many eases to be brouglit about by the reduction of one molecule 

 and tiie oxidation of another. Such actions can progress in solids, liquids, 

 or gases. They may be produced by visible or ultra-violet light, by X rays, 

 by 7 rays, or by /3 rays. They may be sudden when intense radiation acts 

 upon easily clianged substances. Or they may take ages to accomplish, as in 

 the case of mineral changes brought about by the ionising activity of the 

 a rays of radioactive substances acting in the rocks. 



On the living cell y or X rays produce remarkable effects. The study 

 of tliese effects in plants dates back several years. Scliobert, Errera, 

 Molisch, Guilleminofc, and others have contributed to it.^ The rays may 

 retard cell division, and more especially affect the germinating embryo. 



Le Radiinii, vii, 1910, p. 247. 



