4 DISCOVERY AND NATURE OF RADIOACTIVITY 



it will collide with some of the molecules of the gas. As a result of 

 this collision, a unit charge of negative electricity (electron) may be 

 torn from the molecule. The remaining portion of the molecule, by 

 virtue of the loss of the electron, is positively charged. Such a sepa- 

 ration of molecules into negatively and positively charged units is 

 called ionization. Each of the units is a gaseous ion^ and the gas is 

 said to be ionized. 



At low pressures, such for example as obtain in a vacuum tube, 

 the electron is all there is of the negative ion, and the positively 

 charged remainder of the molecule constitutes the positive ion ; * but 

 at atmospheric pressure each of these charged bodies becomes the 

 center of aggregation of several molecules, and then the central 

 charged nucleus, together with the surrounding molecules^ is regarded 

 as an ion. Gaseous ions are positive or negative according to their 

 charge. 



It is essential not to confuse the free gaseous ions with the ions of 

 electrolytes in solution, and the employment of the same term in two 

 senses is, in some ways, unfortunate. Negative gaseous ions are frag- 

 ments of atoms, while the ions of electrolytes in solution result from 

 the splitting up of molecules. The mass of a free negative gaseous 

 ion is about t^.^^ the mass of a H ion in solution (Jones), but they both 

 carry the same kind of a charge. The free negative ions, or elec- 

 trons, are the same as the "satellites" of Kelvin, and the "cor- 

 puscles " or " particles " of J. J. Thomson. Electrons do not behave 

 as a gas. They cling to positively charged bodies, and, if left quiet, 

 settle on the walls of the containing vessel. 



Discovery of X Rays : Three years previous to Thomson's 

 proposal of the term corpuscle, Rontgen -"^-^^ read before the Wiirz- 

 burg Physico-Medical Society his epoch-making communication on 

 the X rays, and later in the same year Perrin^' and Stokes ^^^ showed 

 that X rays are probably electro-magnetic pulses in the ether, and 

 develop at any place where a body arrests the motion of the elec- 

 trons of the cathode rays. This conception was subsequently more 

 fully expressed by Thomson. '^^ Thus when the cathode rays are 

 stopped b}^ the walls of the Crookes tube, X rays result. Here was 

 a new kind of ray that could pass through bodies opaque to ordinary 

 light, and darken a photographic negative. f 



*The mass of positive ions varies with the substances from which thej are pro- 

 duced. 



t The eftect of X rajs on a photographic negative was in reality a later discovery. 



