in Gases by the Aid of Ultra- Violet Light, 393 



ionizing actions are called into play as the distance between 



the plates increases, so that the ratio -^i increases as the 



distance between the plates increases. A few examples of 

 this are given in the table for hydrochloric acid gas. Thus 



in the experiment P = 5'l, X = 875 the ratio — exceeds the 

 ratio — by an amount which cannot be attributed to experi- 

 mental error. Another example is to be found when P = *98 

 and X= 700; the ratios — and - 3 are equal to one another, 

 and are both less than the ratio — . I am at present 



engaged in making some further experiments in this direction, 

 which I hope to have ready soon for publication. 



When the distance between the plates does not exceed a 

 certain limit, depending on the values of X and P, the ratios 



of the conductivities 2 , — , &c are equal, and the inequa- 

 lities n x < n 2 < n % &c. can be accurately explained by attributing 

 the increases in conductivity to the action of the negative 

 ions alone. All the experiments for any gas can then be 



represented by a single curve. The theory shows that - 



X P 



should be a function of — , a being the number of ions 



p ' & 



generated by a single negative ion in travelling through one 



centimetre of a gas at pressure p under the electric force X. 



The values of a can be found from the experimental results 



by substituting the numbers found for n in the formula 



no n<* 



^__ e axL or _o =6 ax-2^ r^Yie points whose coordinates are 



n x n x l 



a. \ 



- and —are marked on the accompanying diagrams (figs. 2 & 3, 



PL IX,), and it can be seen that a single curve passes through 

 all the points. This shows that a y X, and p, as determined 

 experimentally, are connected by an equation of the form 



— ) which affords further confirmation of the theory. 

 p) J 



It was found necessary to give each curve in two parts 

 since the variables extend over such large ranges. 



I have already shown how some physical properties of 

 gases and ions may be deduced from these results, such as 

 the mean free path of an ion, and an approximate value of 

 the size of a molecule. The first of these quantities may be 



