1 8 NUCLEATION OF THE UNCONTAMINATED ATMOSPHERE. 



17. Manipulation Fog limit. The experiments were conducted as 

 follows : Having selected a suitable pressure difference above that at 

 which condensation in dust-free air just begins (usually termed the 

 " fog limit " in the present paper), the dust-free moist air in the closed 

 condensation chamber at atmospheric pressure is suddenly exhausted 

 and the corona measured. After all fog has subsided the exhaustion 

 cock is closed and the filtered air very slowly admitted. The opera- 

 tions are then repeated, allowing time (about 2 to 3 minutes) for 

 saturation. Under all circumstances the treatment for large and 

 small coronas was identical. 



In the given apparatus condensation in dust-free moist air began at 

 the pressure difference, 8^ = 22.5, corresponding to the volume expan- 

 sion of about 1.43. The pressure difference usually applied in the 

 experiments was S/> = 3i.2, and the volume expansion 1.72. 



18. Alternations of large and small coronas (periodicity of inferior 

 and superior coronas). The small coronas are usually sharp, but the 

 large coronas appear blurred and filmy, accompanied with much rain. 

 Remembering that all operations are conducted in a way strictly the 

 same, table 10 (pp. 19-20) shows the coronas seen in the successive 

 exhaustions. The angular diameter or aperture is sin ^>/2 = ^/6o, or 

 nearly 4> = sl3o. The eye at the goniometer was about 40 cm. from 

 the axis of the condensation chamber (placed as close as possible to 

 insure clearer vision) and the source of light 250 cm. beyond it. 

 Observations were made along the axis of the cylinder, placed hori- 

 zontally. The number of nuclei per cubic centimeter of the exhausted 

 air will be denoted by n, while A'' shows the number per cubic centi- 

 meter of air at normal pressure. The reduction of n to A 7 where it is 

 not essential will often be omitted. 



In the case of 2-rninute periods between the exhaustions the perio- 

 dicity is maintained without exception (fig. 14). For brevity let the 

 smaller coronas be called inferior, the larger coronas superior. Fre- 

 quently a very small inferior corona evokes a relatively large superior 

 corona, or larger inferior coronas are followed by smaller superior 

 coronas ; but this is not always the case. As a more general rule, if 

 the aperture is intermediate between the inferior and superior coronas, 

 the succeeding corona is of the same size, and oscillation terminates. 

 In part III, for an accidentally more rapid influx than the exceedingly 

 slow influx of filtered air in the earlier parts of table 10, this is initially 

 the case, but the oscillation is soon reestablished. In part IV, the water 

 was shaken so as to wet the glass sides of the chamber, but without 

 effect on the oscillation (fig. 15). In part V of table 10, the original 

 periodicity is again wiped out by accidental influx of much air through 



