ALTERNATIONS OF APERTURE. 31 



larger and smaller precipitations, the cases may be interpreted in suc- 

 cession as follows : 



a. The superior coronas carry down more moisture and should be 

 followed by even larger coronas, and vice versa ; but after the fog par- 

 ticles producing the superior coronas are precipitated, the supersat- 

 uration possible for the given pressure applied no longer catches 

 the small nuclei. Hence the inferior coronas appear in succession. 

 Hence, also, apart from what may be time errors in opening the stop- 

 cock, very large pressure differences tend to wipe out the oscillation, 

 as all nuclei are caught. 



b. The ratio of i : 2 for coronal apertures and of i : 8 for the volumes 

 of fog particles seems out of keeping with the slight differences of 

 supersaturation instanced in section 28; but this is again a question of 

 catching the group of smaller nuclei. 



c. The phenomenon is much too definite an oscillation of aperture 

 between ^ and 2s (nearly) to be referable to an irregular cause like 

 deficient supersaturation ; but the two types of nuclei admit of a wide 

 range of saturation, as long as there is a correspondingly wide dif- 

 ference in the sizes of nuclei. 



d. A series of minor observation are favorable to the hypothesis of 

 residual undersaturation ; as, for instance, the eventual coalescence of 

 the aperture curves of the superior and the inferior coronas ; the dew 

 effect ; the fog effect and shaking ; the fact that very small inferior 

 coronas are followed (ccet. par.} by large superior coronas, while the 

 latter are followed by large inferior coronas, etc. 



e. Finally, while superior coronas are followed by inferior coronas, 

 and vice versa, mean coronas follow each other. 



30. Nucleation. The values of the nucleatiou (number of nuclei 

 per cubic centimeter) of the inferior and the superior coronas naturally 

 present a more striking contrast, since the third power of aperture is 

 involved. Otherwise but few new results are to be inferred from them. 

 If the long series of table 10, part II, be taken, which contains the 

 data of twenty successive alternations, the average inferior nucleations 

 are 11,800, and the average superior nucleations 94,000, supposing, of 

 course, that the precipitated water is the same in both cases and that 

 it is all condensed on the available nuclei. In other words, if the two 

 cases are identical, the superior coronas correspond to a number of 

 nuclei 8 times greater (frequently larger than this in the other obser- 

 vations) than the inferior coronas. As this explanation is the more 

 probable, it follows that the nuclei can not be regarded as positive and 

 negative ions. They are rather the groups of large and small nuclei 



