A CONTINUOUS RECORD OF ATMOSPHERIC NUCLEATION. Ill 



other of the early coronas are very apt to fade into a coarse white reddish corona. 

 This is the evaporation of the smaller particles into the larger, which accounts, 

 moreover, for the loss of nuclei during the first precipitation, to be caught in 

 subsequent exhaustions. The successive coronas in a series gradually become 

 sharper and the larger particles more uniform, but extrerrely fine particles are 

 still present even when one approaches the normal coronas. The fine particles, 

 however, belong to coronas so large and diffuse that their coronal effect scarcely 

 modifies the strong coronas of the large particles even before the former vanish 

 by evaporation. 



When I first observed these different sizes of drops caught on a single plate, 

 it seemed not in probable that a difference of the condensational effect of the 

 negative and the positive ions might here be actually in evidence; but as all 

 intermediate sizes are present at the outset, and particularly as large and srrall 

 droplets still appear together long after all electrification has certainly vanished, 

 this conclusion is not warranted. What continually favors uniformity is sub- 

 sidence of fog. As the phosphorus nuclei are graded, it is probable that the 

 very fine droplets are due to the initial or primitive nuclei from which the larger 

 nuclei have grown by coalescence; or the fine droplets may be due to air nuclei 

 associated with the phosphorus nuclei. All this will appear in the more minute 

 photographic study of the subject detailed in the next section, and it will be 

 further interesting to decide whether the nuclei generated by the X-rays are not 

 also graded below a certain usually much smaller maximum diameter. That 

 this maximum diarr eter will increase with the lapse of time allowed for coherence 

 may be inferred. 



The coarse and washed type of coronas obtained with nuclei produced by 

 the X-rays is evidence of graded size, while the fog particles, so far as I have yet 

 caught them, arc of varied dimensions. In these cases the X-rays reached the 

 inside of the condensation chamber through its waxed wood walls lined with wet 

 cloth. To obtain a fairly strong and large corona an exposure to the rays lasting 5 

 to 10 minutes was needed, as the radiation was not very intense. In this interval 

 the original extremely small nuclei are probably undergoing continuous growth, 

 for instance, by cohering, so that on exhaustion particles of all sizes are revealed. 

 In addition to the ragged coronas there is copious rain. Under these circum- 

 stances it seems reasonable that the time loss of nuclei must at the outset be 

 proportional to the square but finally to the first power of the number, assuming 

 that eventually the large nuclei do most of the catching. 



II. MICRO-PHOTOGRAPHY OF FOG PARTICLES. 



8. Preliminary. In the preceding section ' I described a series of experi- 

 ments in which the diameters of fog particles were microscopically measured, 

 directly. In the present section these particles are micro-photographed, and 

 the negatives subsequently measured. The results, though interesting as a whole, 

 1 See also American Journ. (4), xvn, p. 160, 1904. 



