OF FLAMES CONTAINING SALT VAPOUES. 
525 
Ptontgen rays is greater than that of the positive ions in air by about 25 per cent. 
McClelland has found the same thing for the ions in the gases coining from flames, 
and Rutherford has shovm that the same result holds good for the ions produced 
by uranium radiation. 
In the flame the negative ions of alkali salt vapours move 17 times as fast as the 
positive ions, and in air at 1000° C. 3’6 times faster with alkali salts, and 7 times 
faster with salts of Ba, Sr, and Ca. 
It seems reasonable to suppose that since the ions in each of the three classes, viz. : 
1 Negative ions, 
2 Positive ions of alkali metal salts, 
3 Positive ions of Ba, Sr and Ca salts, 
have equal velocities, they are equal clusters of atoms. It thus appears that ions 
which in solutions have equal charges, have equal velocities in the gaseous state. 
This points to the conclusion that the size of the cluster of atoms forming a gaseous 
ion, depends, at a given temperature, only on the charge on the ion. Those ions, 
therefore, which have equal charges, have also equal velocities in the same medium. 
(7.) Conclusion. 
Since the ionisation on which the conductivity of the salt vapour depends takes 
place entirely at the surfaces of the glowing electrodes, there is therefore at the 
surfaces a thin layer in which very raj)id ionisation and recombination are going on. 
The number of ions dragged out from the surface of an electrode will depend on 
the slope of potential at the surface, and if this is great enough to drag out all the 
ions of one sort before they can recombine the current will be as great as possible. 
Owing to the much greater velocity of the negative ions they will be far more easily 
dragged out than the positive ions, so that unless the slope of potential is great 
enough to drag out all of either kind of ions, the current from an electrode with a 
given slope of potential at it will be greater when the electrode is negatively charged 
than when it is positively charged. 
Consider the case in which one electrode is white hot and the other comparatively 
cool, so that little or no ionisation occurs at it. In this case oidy one kind of ions 
will be present in the space between the electrodes, viz., those of the same sign as the 
hot electrode, so that there will be a charge in the gas which will diminish the slope 
of potential near the hot electrode and increase it near the cool electrode. The 
experimentally-determined slopes of potential with one electrode cool show this effect 
very clearly. 
In this case, in which the fall of potential is nearly all at the cold electrode, the 
smallness of the potential gradient at the hot electrode is not favourable to the 
attainment of the saturation value of the current, and the current E.M.F. curves 
