FOG LIMIT. 5 



This indicates that at a pressure difference of about Sf> = 22 cm. for 

 the given apparatus and dust-free moist air, spontaneous condensation 

 with vanishing coronas begins on sudden cooling and that thereafter 

 the coronas increase regularly. This pressure, 8p , will be usually 

 referred to as the "fog limit." 



In corroboration with the preceding, similar experiments were tried 

 with an instantaneous valve, opened with a hammer, and having a 

 clear bore of over i inch. The results shown in table 5 were identical 

 on both sides, but unexpectedly irregular, the only explanation for 

 which might seem attributable to a possibly unequal degree of sudden- 

 ness in opening the valve. But this is not the case; for alternations of 

 large and small coronas in dust-free air, such as are here imperfectly 

 shown, may be kept up indefinitely 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, can not be 

 called in question, as I shall show in Chapter II. 



TABLE 5. Spontaneous condensation of saturated air. Angular diameter 0=5/30. 



Press, diff., 5/= ig cm ig. 4 cm 2i.4 cm 24 



5 = 2.3 3.4 3.3 4.4 



Repeated, s= o 2.1 2.0 2.5 



s= o o 3.0 4.3 



S = O O 2.O 3.5 



* = o 3.5 3.3 



5= 2.2 3.3 



Mean, \ s= 3-6 



( n- o o 7,600 21,000 



5/ <20 em , W=0 



For 8/> = i9.4 and below, therefore, no nuclei appeared after thor- 

 ough cleaning. For 8/> = 2o cm. and above, i. <?., at a somewhat 

 lower pressure difference than before in consequence of more rapid 

 exhaustion, spontaneous condensation begins. The large coronas 

 are blurred. Hence in neither case will air nuclei be caught at 

 Bp ij cm., in the given apparatus. 



<T. Possibility of producing nuclei by sudden intense exhaustion.* The 



condensation of the moist air in the absence of foreign nuclei may 

 be considered as due to the spontaneous nucleation of the air, the 

 available nuclei increasing in abundance as with increasing pressure 



* Investigations on the spontaneous condensation of moist air were first suggested 

 by myself, in Bull. U. S. Weather Bureau, No. 12, 1893, pp. 13 and 48. They have 

 since been fully treated in the masterly work of C. T. R. Wilson, Trans. Royal Soc. 

 Lond., vol. 189, pp. 265, 307, 1897 ; ibid., vol. 192, pp. 403-453, 1899. 



