A CONTINUOUS RECORD OF ATMOSPHERIC NUCLEATION. ill 



Other o£ the carl}- 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 

 shariDcr 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 diftuse 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 partictilarly as large and small 

 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 phosphoms 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 inay be due to air nuclei 

 associated with the phosphoiais niiclei. All this will appear in the m.ore minute 

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

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

 also graded below a certain ustially mtich smaller maximum diameter. That 

 this maximum dian" 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 b}' 

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

 caught them, are 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 oVjtain 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 continvious 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. Pniiniijiary. — 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 restilts, though interesting as a whole, 

 ' See also American Jonrn. (4), xvu, p. 160, 1904. 



