NUCLEATION IN LAPSE OF TIME. 33 



be subject to something resembling saturation, whereby ions pass into 

 persistent nuclei when the ionization pressure reaches a certain limit 

 very much as a vapor condenses. 



33. Dependence of efficiency of fog chamber on the size of the exhaust 

 pipes. If a (long) cylindrical fog chamber, whose contents are not 

 much in excess of 6,000 c. cm., be exhausted into a vacuum chamber 

 whose contents exceed 100,000 c. cm., through a passage-way 50 cm. in 

 length and about 2.5 cm. in bore, many of the experiments showing 

 the properties of ions and of the colloidal nuclei of dust-free air are con- 

 sistently producible, provided the plug exhaustion stopcock (opened 

 quite and as quickly as possible) has a wider bore than the pipes. With 

 exhaust pipes of this size, however, the apparatus is as yet very far 

 from its limits of efficiency, and the largest or terminal corona obtainable 

 in case of the colloidal nucleation of dust-free air is, even at the highest 

 exhaustion, the intermediate green-blue-purple type. This leaves the 

 whole order of large coronas outstanding, though the order may be 

 entered if the air is intensely ionized. It follows that efficiency actually 

 stops short because of the smallness of the colloidal nuclei. 



The curves showing the nucleation entrapped in case of continually 

 increasing exhaustion all have a common feature; there is an initial 

 low branch of small variation attributable to ions; an intermediate 

 branch showing steep ascent, due to colloidal nuclei; a final asymptotic 

 branch, corresponding to the terminal corona and due to the loss of 

 efficiency of the apparatus. The middle branch in question, or line of 

 representative colloidal nuclei, if it appears at all, has almost the same 

 slope and position (relative to the field as a whole) for all fog chambers, 

 whether their efficiency be small or great. The fog chamber ceases to 

 act almost abruptly; in other words, the terminal corona or largest 

 corona producible as the drop of pressure continually increases, appears 

 in any fog chamber as an almost sudden departure from the representa- 

 tive lines. This corona is larger or smaller, the asymptote higher or 

 lower, as the fog chamber is more or less efficient. 



34. Invariable character of colloidal nucleation in the lapse of time. 



The terminal corona (in a given type of apparatus), if not too small 

 (Chapter VI), is a fixture as to size in the lapse of time (months). It 

 does not vary appreciably with the departure from normal ionization, 

 to which atmospheric air is incident. Probably the number of available 

 colloidal nuclei is too enormous, as compared with the small variations 

 in number of the relatively large ions, to admit of an observable effect. 



