C. Barns — Large and Small Coronas. 353 



9. Fog-limits. — An interesting feature of these results are 

 the fog-limits or pressure differences at which condensation in 

 dust-free air just commences. In spite of the different sizes 

 of apparatus and valves used, the fog-limits are about the 



same, viz. : 









Sp = 22-23 



Apparatus, 



wood, v/V = *13 Valve, plug. 



21-5- 



u 





glass, v/V = *06 " clapper. 



22-23 



a 





" " plug. 



21-23 



a 





" u clapper. 



22-23 



a 





" " plug. 



These results 



are sui 



fprising, inasmuch as the effect of the 



volume ratio of fog and vacuum chambers and the valve effect 

 would naturally be looked to as productive of larger differ- 

 ences. With other apparatus (this Journal, vol. xix, p. 175) 

 the data were 



Bp = 22 Apparatus, wood, v/V = -7 Valve, plug. 



20 " " -7 " clapper. 



Thus the supreme importance of mere rate of exhaustion may 

 well be called in question until more definite results appear ; 

 for with so large a difference of volume ratio v/V, of valve 

 obstruction, etc., the essential features should appear more 

 clearly. One may note that if colloidal molecules (extremely 

 fine nuclei) pass through the filter, these would capture most 

 of the moisture on condensation. It is possible therefore that if 

 the filter is dispensed with and a closed vessel used, larger coronas 

 will appear at smaller pressure differences for this reason. 



10. Alternations of large and small coronas. — Effectively, 

 the case of oscillation is one in which the large sparsely dis- 

 tributed fog particles emit more nuclei and the very abundant 

 small fog particles fewer nuclei ; i. e., the phenomena may be 

 looked upon as though the nuclei were generated during the 

 growth of the fog particles. This plausible explanation, how- 

 ever, is not easily maintained ; for the emission would have to 

 be as the growth of surface, in other words as the volume, and 

 the number of particles varies inversely as their volume. A 

 counter supposition may be hazarded to the effect that the fog 

 particles of large coronas absorb more nuclei because of their 

 abundance than the fog particles of small coronas. But the 

 period of suspension of particles is too short to be of moment. 



If negative ions are more active as condensation nuclei than 

 positive ions, the results observed may be tentatively grouped 

 in according with the following scheme (see diagram, p. 354). 



Let the ions be originally neutral as a whole, and suppose, as 

 in case 1, that the negative ions are first precipitated. In the 

 interval between this and the next exhaustion fresh ions are 

 generated or taken in through the filter, as shown in case 2. 

 If these negative ions partially neutralize the positive ions left 

 over in case 1. the second precipitation takes place on the 



