CHAPTER II. 



THE CHANGE OF THE VAPOR NUCLEATION OF DUST-FREE WET AIR IN 

 THE LAPSE OF TIME, TOGETHER WITH THE EFFECT OF THE LIMITS 

 OF PRESSURE BETWEEN WHICH A GIVEN DROP TAKES PLACE ON 

 THE EFFICIENCY OF THE FOG CHAMBER. 



11. Introduction. Recently* I published certain results which showed 

 (apparently) that the colloidal nucleation of dust-free air varies peri- 

 odically in the lapse of time in a way closely following the fluctuations 

 of the barometer. This nucleation (particularly when the larger groups 

 of nuclei lying near the region of ions are taken into consideration) is a 

 maximum when the barometer is a minimum. The development of 

 the investigation was peculiar. At the outset the data appeared like 

 an immediate confirmation of Wood and Campbell's! discovery, 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 virtually 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 apparent direct effect of fluctuations 

 of the barometer. Nucleation of dust-free air increased when the barom- 

 eter decreased, and maxima of nucleation were apt to coincide with 

 minima of the barometer. Such a result, whether direct or indirect 

 (removal of radioactive 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. Unfortunately 

 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 (one 

 with 2-inch, the other with 4-inch exhaust pipes), adjusted for different 

 sizes of coronas and accentuating the barometric correction, the vari- 

 ations in one vessel might be made to show a tendency to follow the 

 barometer, whereas the other departed from it. Table 6 and fig. 4 give 

 an example of such a case, where 8p is the observed fall of pressure 

 (P PS)* P the pressure of the fog chamber before, p 3 the pressure after 



*Carnegie Institution of Washington Publication No. 62, chap, vi, 1907. Cf. Science, 

 xxm, p. 952, 1906; xxiv, p. 1 80, 1906. 



fWood and Campbell: Nature, LXXIII, p. 583, 1906. 



