C. Barus — Large and Small Coronas. 355 



In the above tables a was usually less than 10 cm (glass fog 

 chamber), making the condition correspondingly favorable. 



Hence by diffusion alone there should be saturation after 

 two to three minutes even at the most distant (middle, x = a/2) 

 plane, to within a few per cent ; for the central layer is prob- 

 ably always more than half saturated at the outset. In addi- 

 tion to diffusion, however, there is marked convection due to 

 the lightness of water vapor. At the same time there is no 

 evidence that the more numerous but small drops of the 

 superior coronas carry down a sufficient excess of water ; nor 

 are the coronas, though blurred otherwise distorted, as they 

 would be for a definite diffusion gradient. 



12. Continued. — Assuming however that undersaturation 

 does occur and is oscillatory as the result of successive larger 

 and smaller precipitations, the cases may be interpreted in 

 succession as follows ; 



a. The superior coronas carry down more moisture and 

 should apparently be followed by even larger coronas; and 

 vice versa : but after the fog particles producing the superior 

 coronas are precipitated, the supersaturation possible for the 

 given pressure difference applied no longer catches the small 

 nuclei. Hence the inferior coronas appears in succession. 

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

 the stopcock, very large pressure differences tend to wipe out 

 the oscillation as all the nuclei are captured. 



b. The ratio of 1 : 2 for coronal apertures and of 1:8 for 

 the numbers of fog particles seems out of keeping with the 

 slight differences of supersaturation instanced in 13 ; but this 

 is again a question of catching the smaller nuclei as a group. 



c. The phenomenon is much too definite an oscillation of 

 aperture between s and 2s (nearly) to be referable to an irreg- 

 ular 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 difference in the sizes of nuclei. 



d. A series of minor observations are favorable to the hypo- 

 thesis of residual undersaturation ; as for instance, the even- 

 tual 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 (caet. 

 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. * 



13. The values of the nucleation (number of nuclei per 

 cub. cm.) of the inferior and the superior coronas naturally 



* I have since proved that periodicity is due to the formation of water 

 nuclei by evaporation. On these the inferior coronas condense. Points a 

 to e then follow. The ions become solutes. 



