424 Dr Frank Horton, The ionisation produced by certain, etc. 
is much larger than at ordinary temperatures, and the pressure of 
maximum current increases with both these quantities. I think, 
therefore, that the shape of the curves given by pure platinum 
and by a clean Nernst filament can be explained by ionisation by 
collisions coming in at certain pressures. 
The effect of sodium phosphate is probably twofold: (1) it 
increases the formation of positive ions at the surface of the 
anode, and (2) it changes the nature of the gaseous material 
through which the discharge takes place; for the space round the 
anode now contains molecules of volatilised salt, or the products 
of dissociation of that salt. The pressure of maximum current 
is now much lower than before, and if ionisation by collisions is 
the cause of the existence of this maximum, it follows that the 
vapour now surrounding the anode is not so easily ionisable by 
collisions as was the gas previously in contact with it. On this 
view the increase of energy required for ionisation must be 
considerable to account for the large decrease in the pressure of 
maximum emission. If we may assume that an increase of tem- 
perature of the anode increases the percentage of difficultly 
ionisable molecules surrounding it (by increasing the volatilisation 
or dissociation of the salt) to a greater extent than is necessary to 
counterbalance the effect of the increased mean free path, we 
should expect that the higher the temperature of the anode the 
lower would be the pressure at which the maximum emission 
occurs. This is what is actually found to happen when series 
of observations of the positive emission are taken at different 
temperatures. In the case of the negative emission from glowing 
solids, the pressure of maximum current increases with the tem- 
perature, a fact which is explained by the larger mean free path 
in the surrounding gas at higher temperatures. If the above 
explanation of the effects obtained with the positive ionisation is 
correct, it follows that the difficulty in ionising the salt vapour 
(or its dissociation products) 1s experienced by the positive ions 
but not by electrons. 
So far nothing has been said about the increase in the positive 
emission from the clean filament as the pressure is reduced below 
1 mm. An exactly similar increase was noticed when pure 
platinum was being experimented upon, and it has been attributed 
to the mercury vapour which comes over from the pump. It is 
intended to test this view by taking precautions to prevent mercury 
vapour from entering the discharge tube. 
The author wishes to acknowledge his indebtedness to the 
Government Grant Committee of the Royal Society for the means — 
of purchasing some of the apparatus used in these experiments. 
