6'2i Mr. N. R. Campbell : Experiments on 



General Conclusions. 



It will be at once noticed that the minimum expansion 

 required to give a cloud while the discharge is passing has 

 the same value for both signs in air and in nitrogen, and for 

 the positive discharge in hydrogen and oxygen, and that this 

 value agrees very approximately with that found by C. T. R. 

 Wilson for condensation on negative ions in these gases, 

 namely^ 1*25. We may therefore conclude that in these 

 cases the largest nuclei produced by the brush are the 

 negative ions. 



The corresponding value for the negative brush in hydrogen 

 is somewhat less. This agrees with Mr. Wilson^s r.^sult 

 that the expansion required with the positive discharge is 

 slightly greater than that required with the negative discharge; 

 which he attributes to the greater fog-dispersing power of 

 the former, since this property renders very fine showers 

 difficult to observe. It is remarkable, however, that in these 

 experiments the value for the negative discharge is less than 

 1'25, while that for the positive is only slightly greater. 

 Hence Mr. Wilson's explanation seems hardly admissible. 

 Slight impurities in the gas, which are notably hard to 

 remove in the case of hydrogen, might cause the peculiarity, 

 but it is surprising that they should give so consistent and 

 so definite a value. In this connexion it may be noted, that 

 the experiments on the negative brush seemed always more 

 liable to disturbance by slight impurities than those on the 

 positive. 



It is possible that the slightly smaller value of — for the 



^1 

 negative discharge may be due to " growth ^' of the ions 

 w^hile diffusing from the point to the sides of the bulb : this 

 is hardly probable, however, for it would be expected that 

 the iDns would diffuse faster in hydrogen than in air, and 

 hence would have less time to grow. 



The irregularities of the results in oxygen, with which 

 may be classed those with magnesium and sodium amalgam 

 in air, call for some explanation. The clouds produced in 

 these instances were not visible at the ordinary pressure, 

 but never required an expansion greater than 1*08 to ensure 

 their appearance ; between these limits the value varied in 

 a perfectly irregular manner. 



It is probable that this effect may be attributed to 

 the phenomena discovered by Meissner, in connexion with 

 the production of ozone ^, which are generally considered 



* Meissner, Untersadumu iibcr den Sauer stuff \ Iluuiburir, 18133. 



