202 C. Barus — Changes of Colloidal Nucleation. 



Art. XX. — Changes of the Colloidal Sucleation of Dust- 

 free Wet Air in the Lapse of Time ; by Carl Barus. 



1. Introduction. — The development* of the present inves- 

 tigation was peculiar. At the outset the data appeared like 

 an immediate confirmation of Wood and Campbell'sf dis- 

 covery, which had then just been announced. Maxima of 

 colloidal nucleation appeared where Wood and Campbell had 

 found minima of ionization, and vice versa. By supposing 

 that the ions which are larger than the colloidal nuclei, capture 

 most of the precipitated water, the two sets of results would 

 be mutually corroborative. 



Later this cosmical feature of the phenomenon became of 

 secondary importance as compared with an apparently direct 

 effect of fluctuations of the barometer. Nucleation of dust- 

 free air increased when the barometer decreased, and maxima 

 of nucleation were apt to coincide with minima of the bar- 

 ometer. Such a result whether direct or indirect (removal of 

 radio-active matter from porous earth accompanied by falling 

 barometer) would have been of considerable importance, and 

 great care had to be taken in the endeavor to verify it. Unfor- 

 tunately the correction to be applied for barometer fluctuation 

 in its effect upon the aperture of the coronas, was in the same 

 sense and very difficult to estimate : and in fact upon using 

 two fog chambers side by side, adjusted for different sizes of 

 coronas and accentuating the barometric correction, the varia- 

 tions in one vessel might be made to show a tendency to follow 

 the barometer whereas the other departed from it. The dis- 

 crepancy in these results may have been an overcompensation, 

 although all the details of the experiments themselves were 

 gradually more and more fully perfected ; or the rise in the 

 region of ions may just have balanced the decrease of the num- 

 ber of efficient colloidal nuclei due to the increase of the former. 

 In fact the region where ions predominate may rise, while the 

 regions where the colloidal nuclei are more important may 

 correspondingly decrease, producing a diminished slope of the 

 initial part of the graph, such as is often actually observed. 

 It is necessary therefore to inquire even more carefully into the 

 errors involved, to investigate some data or invariant which if 

 kept constant will mean a corona of fixed aperture in the 

 given apparatus, unless there is actual radiation in varying 

 amount entering from without. 



I purpose therefore in the present paper to study the same 

 subject for an artificial barometer ; in other words, to accentu- 



* Science, xxiii, p. 952, 1906 ; xxiv, p. 180, 1906. 

 f Wood and Campbell, Nature, lxxiii, p. 583, 1906. 



