538 Sir J. J. Thomson on the Structure of 



of these two were removed, the two cells would be connected 

 only by a single electron which might quite likely be 

 insufficient to hold them together and the molecule would 

 dissociate. This, however, would only happen if the electron 

 removed were one of two special electrons ; if any other of 

 the 14 electrons were removed the connexion between the 

 cells would be unimpaired and the molecule, though un- 

 saturated, would be stable : as a matter of fact, positively 

 electrified chlorine molecules are found in the positive rays. 



It is when we consider systems with more than one posi- 

 tive charge that the difference between atoms and molecules 

 becomes most apparent; for while the parabolas corresponding 

 to doubly charged atoms are to be found on nearly every 

 positive ray photograph, those corresponding to doubly 

 charged molecules though not unknown are rare. If the 

 molecules were those of elementary gases, the parabola of the 

 molecule with the double charge would coincide with that of 

 the atom with a single charge, and so might escape notice ; 

 this, however, would not apply to molecules of compound 

 gases, and even in the case of elementary gases the existence 

 of doubly charged molecules would modify the appearance of 

 the parabola due to the singly charged molecule in a way that 

 would lead to their detection. This very striking difference 

 between atoms and molecules is, I think, due to the fact that 

 it is the electrons which hold together the atoms in a molecule; 

 they are, in fact, structural, and in general, when more than 

 one of them is removed, the structure is weakened to such an 

 extent that the molecule splits up and ceases to be a molecule. 



The mechanism by which the double charges are produced 

 may be one which is much more likely to produce doubly 

 charged atoms than doubly charged molecules. If the double 

 charge was due to the system losing one electron by one 

 collision and a second one by a subsequent collision, we 

 should not expect to find the marked discrepancy between 

 the numbers of atoms and molecules with double charges. 

 There is, however, ample evidence from the positive rays 

 that the atoms acquire their double charges by a single 

 operation, and not in this way. This operation may well be 

 the breaking up of a molecule containing the atoms. Thus, 

 for example, if from a molecule of oxygen an octet were to 

 break away it would leave behind it an atom of oxygen and 

 four electrons ; as the oxygen atom carrries 6 positive charges, 

 the system would have a double charge. Thus if the double 

 charge, as seems probable, is due to the breaking up of 

 molecules, we should expect to find the atoms rather than 

 the molecules in possession of these charges. If one of the 



