A CONTINUOUS RECORD OF ATMOSPHERIC NUCLEATION. 121 



21. Summary. The curious state of the case mentioned is not reassuring. 

 Briefly, the photographic w-values and the photographic values for diameter, 

 d, do not make an incompatible system, though both differ materially from the 

 coronal values, the former (d) being larger, the (ri) smaller. If one ascribes the 

 large visible diameters to adhesion, and the small numbers counted to evaporation, 

 the compatibility of the two sets of independent data (d and n) is not explained. 



I have therefore concluded that the results in question suggest that the 

 smaller particles evaporate rapidly into the larger. Hence while the mass of 

 water precipitated per cubic centim. remains constant, the diameter of the fog 

 particles soon increases while their number decreases (by evanescence of the 

 smaller) in such a way that n X d 3 remains very nearly constant throughout. 

 In place of evaporation, capillary coalescence of the initial very minute droplets 

 is an even greater probability ; and such coalescence I have often seen under the 

 microscope on observing dew droplets ' very fine and close together. It is rather 

 interesting that the evidence in favor of this complicated behavior is so strong. 



That such evaporation occurs appreciably in the coronas is not probable, be- 

 cause in a good apparatus they retain their character during the whole of the subsid- 

 ence. The plate of glass being slightly warmer contributes to the effect observed. 



Hence neither the micrometric nor the photometric method can be relied 

 upon for undistorted results. Whenever graded particles are present the evi- 

 dence must be sought in the washed and blurred coronas. If the coronas are 

 clear and multi-annular, the gradation observed under the microscope must be 

 a secondary effect not present in the coronas themselves. 



III. RESULTS FROM SUBSIDENCE. 



22. Object and method. The difficulties mentioned in the last sections in- 

 duced me to look for corroborative data in the evidence obtainable from special 

 experiments with subsiding fog particles. These are easily made in the 

 apparatus for measuring atmospheric nucleation as described in Chapter 

 VIII, Fig. i. The observer, provided with a stop-watch, simply determines 

 the time during which the straight horizontal line of fog descends a given dis- 

 tance, say 5 centimeters, from the top. In other cases there were two marks 

 between which subsidence was noted. 



23. Results. In the experiments given in the following tables, the watch 

 was started simultaneously with the exhaustion and stopped when the given 

 fall of fog line had been reached. The equations are then successively 



io 4 d = i8l/X ds = a, n = g/(d 3 X io 6 ) = i55o/V /2 , 



where n is the number of particles per cubic centim., d the diameter of each, 

 v the rate of subsidence. 



' If a microscope plate is dipped into thin, rapidly drying methyl alcohol varnish, a milky 

 deposit of dew is often seen on the film when solidifying. This film behaves under the micro- 

 scope as stated in the text. 



