Rays of Positive Electricity. 247 



exception of the hydrogen molecule, there is not one in 

 which the molecule is intact, while many molecules are found 

 among the ions in the primary rays ; for example, the 

 molecules of hydrogen, oxygen, marsh-gas, not to mention 

 those of the monatomic gases like helium and mercury 

 vapour. Again, the ions in the secondary rays carry in 

 many cases more than one unit of charge ; thus, for example, 

 we have C ++ , N ++ , Hg ++ , He ++ , suggesting that the posi- 

 tively charged particles when they collide with a molecule 

 in many cases detach more than one corpuscle from it. 



It is very interesting to find that in the primary rays 

 several different types of ions are found even in elementary 

 gases like hydrogen or oxygen, for, as we have seen, we 

 find both the atoms and the molecules amongst the primary 

 rays in these gases. These ions are supposed to be due to 

 the bombardment of the molecules by the cathode rays. 

 How is it, then, that when we expose the molecules of a gas 

 to bombardment by cathode rays we get two types of ions, 

 atoms as well as molecules in the gases we have just men- 

 tioned ? It is true that in the dark space next the cathode 

 we have cathode rays with very different velocities ; and one 

 way of explaining the tw 7 o types of ions would be to suppose 

 that when the energy of the cathode rays exceeds a certain 

 value, they split the molecule into atoms when they impinge 

 against it, while the slower cathode rays only succeed in 

 knocking a corpuscle out of the molecule without impairing 

 the cohesion between the atoms. On this view the charged 

 atoms would be produced by the fast cathode rays, the 

 charged molecules by the slower ones. If this were the 

 case, however, we should expect that the lines corresponding 

 to the atom would be shorter than those corresponding to 

 the molecule, as the minimum energy required to produce 

 them is greater than that required for the molecule, and the 

 place where the atoms are produced would therefore be 

 further from the cathode than the corresponding place for 

 the molecules ; as a matter of fact, the lines for the atom 

 are often, though not invariably, longer than those for the 

 molecule. 



Another way in which the different kinds of ions might 

 arise is as follows : — Let us suppose that a diatomic mole- 

 cule is made up of two atoms A and B, and that A is 

 positively and B negatively charged. When the cathode 

 rays pass through the gas they may strike either A or B, 

 and detach a corpuscle. If A is struck, then, after the 

 collision, A has two positive and B one negative charge, 

 together they form a system with a total positive charge o( 



