io8 



A CONTINUOUS RECORD OF ATMOSPHERIC NUCLEATION. 



H 



4- 6 8 10 1Z H 



FIGURE 7. CHART GIVING d IN TERMS OF 5. 



Improved Method. 



5. Number of droplets. The following results were obtained with the 

 definite form of apparatus shown in figure i. A method of estimating the 

 nucleation from a direct count made under the microscope is obtained as follows : 

 Let the plate be so rotated eccentrically as to catch the descending fog particles 

 for a definite interval of time, t. If v be their velocity of subsidence, all par- 

 ticles within a height, h, will be caught, if 



h = vt (i) 



and v = io"(i 2 /3.24 (2) 



where the usual value of the viscosity of air has been inserted. Furthermore, 

 m grams are precipitated per cub. cm. by the given exhaustion, and if n be the 

 nucleation 



n = 6m/ 7r d i . (3) 



Finally, if c is the area of the field seen in the microscope and n' the number of 

 particles falling into this field 



n' nhc. (4) 



From these equations n is obtained by eliminating v as 



Sl~ _ 2.1 I X' 



The values of the constants usually^ adopted were t =30 sec., c = i44Xio~ 6 sq. 

 cm., w' /3 = 2.8 X io" 4 , whence -^=1.75 X n'. The experiments to test this 



