C. Burns — Distribution of Nuclei. 179 



irregular. Alternation of large and small coronas in dust-free 

 air, such as are here imperfectly shown, may be kept up indefi- 

 nitely if strictly identical conditions are retained. Effectively, 

 the large fog particles emit more nuclei, the smaller fewer 

 nuclei for the next condensation in order, everything else 

 remaining the same. The importance of these oscillations 

 about the mean aperture, whether the emission is ionized or 

 not, cannot be called in question, as I shall show elsewhere. 



Table 



5. — Spontaneous condensation of saturat 



ed air. Ang 



ular 









diameter <p 



= s/30. 







Press. 



diff. 



,8p = 



19 cm 



19 . 4 cn, 



21 '4 cm 



9_f cm 







s = 



2-3 



3-4 



3-3 



4-4 



Repeated, 



s = 







2-1 



2.0 



2'5 



u 





s = 











3-0 



4-3 



a 





s = 











2-0 



3-5 



a 





S .— 



— 







3-5 



3-3 



a 





s = 



— 



— 



2'2 



3'3 



Mean 





( n = 





 

 o><20 cra , 











n= 



2-7 

 7,600 



36 



21,000 



For &p = 19"4_ and below, therefore, no nuclei appeared after 

 thorough cleaning. For 8j? — 20 crn and above, i. e., at a some- 

 what lower pressure difference than before in consequence of 

 more rapid exhaustion, spontaneous condensation begins. The 

 large coronas are blurred. 



Hence in neither case will spontaneous air nuclei be caught 

 at &j> — 17 cm , in the given apparatus. 



7. Possibility of producing nuclei by very sudden intense 

 exhaustion. — This condensation of moist air in the absence of 

 foreign nuclei is usually considered due to the spontaneous 

 ionization of the air, the available nuclei increasing in abund- 

 ance, as with increasing pressure differences the sizes of cap- 

 tured nuclei are smaller, until the air molecule itself is 

 approached. It follows then that the normal dust-free air 

 always contains unstable systems. 



Hence the question may well be asked, whether very sudden 

 and intense exhaustion may not itself possibly be productive 

 of nuclei. Thus if an unstable molecular configuration is just 

 about to break down, it is conceivable that the tendency to 

 break-down is accentuated by the violent treatment in question. 



We made some experiments on this subject,* using a pres- 



* Investigations on the spontaneous condensation in moist air were first 

 suggested by C. Barus, in Bull. U. S. Weather Bureau, No. 12, 1893, pp. 

 11-14, 48. They have since been fully treated in the masterly work of O. T. 

 R. Wilson, Trans. Royal Soc. Lond., vol. 189, pp. 265-307, 1*897; ibid., vol. 

 192, pp. 403-453. 1899. 



