NUCLEATION CONSTANTS OF CORONAS. 45 



minute and kept open for 5 seconds; it was then closed until the end 

 of the minute. Hence [p 2 ] is the isothermal pressure observed in the 

 fog chamber under the given conditions, determining the density of air 

 and the nucleation left after each exhaustion. The ratio is therefore 



(i) 



where TT is the vapor pressure at the given isothermal temperature T of 

 observation. 



As soon as the exhaust cock was closed the filter cock of the fog 

 chamber was opened, in order to evaporate the fog particles with the 

 least amount of subsidence or other loss. Observation of aperture was 

 made during the 5 seconds in question. 



The relative number of nuclei for a series of coronas of decreasing 

 aperture is obtained in this way. It is furthermore necessary to stand- 

 ardize one of the coronas absolutely. This was done as described in the 

 earlier work (Smithsonian Contributions, No. 1651), and, if d denotes 

 the diameter of the fog particles and 5 the chord of the angular diameters 

 (f> of the corona observed with a goniometer with a radius of 30 cm., 



2 sin (f)/2 =5/30 (2) 



* O \O / 



was accepted when the eye and the source of light were at distances 

 Z} = 3o and 250 cm., respectively, on opposite sides of the fog chamber. 

 With a constant a selected we may then compute the nucleation n' 

 for the smaller white-centered or normal coronas as 



n' = s* ^ 



where m is the amount of water precipitated per cubic centimeter in the 

 exhausted vessel and n' the number of nuclei per cubic centimeter so 

 computed. The theory of diffraction would give a collateral approxi- 

 mation 



6m m 



26. Equations and corrections. In the present experiment no cor- 

 rection was made for the time loss of nuclei, for convection losses 

 during influx and efflux (vortices washing against the walls of the 

 vessel), nor for evaporation loss (loss of water nuclei on evaporation such 

 as occurs with ions but not with solutional nuclei like those here pro- 

 duced by phosphorus, etc.). The justification of this was tested by 

 making series of measurements with widely different exhaustions, 

 [p 2 ], both as to the amount of the latter and number of exhaustions in 

 the series, as will be shown. 



