A CONTINUOUS RECORD OF ATMOSPHERIC NUCLEATION. 113 



accompanies the deposition of the fog particles on the film of viscous varnish, 

 inasmuch as they leave a permanent impression. In other words, the plate be- 

 comes more or less permanently pitted after the fog particles are gone, appearing 

 washed or dull to the naked eye, and not regaining the clear state until after the 

 lapse of 12 hours or more. The effect occurs only in the case of fog deposition 

 and is never present on the plate in the absence of a precipitate of fog. Hence 

 it cannot be an air bubble effect. 



The cause is probably to be associated with surface tension since the weight 

 of the particle is negligible. If the particles were to break through and reach 

 the bottom of the film of varnish, there is no obvious reason why the pitting 

 should vanish like a viscous phenomenon in the course of time. 



It is conceivable (Fig. 8) that the surface tension of the varnish is locally 

 lessened by slight admixture, or at least the proximity of water, and that an 

 alveolar structure of the surface is the result. In certain slides (No. 22) the 

 presence of these "craters," as they may be called, seems to be clear in the pho- 

 tograph. At other times droplets shrinking in their cavity by evaporation 

 were actually observed. But the phenomenon is rare and observation there- 

 fore uncertain. 



1 1 . Dew. When the plate is dry, the beginnings of the formation of dew 

 on its surface are apparent after long exposure. The dew particles are very 

 fine even as compared with fog particles, the former as observed lying within 

 .0001 centim. Their number is enormous, aggregating to fully 2 million or 

 more per square centim. of the surface (slide No. 27). They do not further 

 seem to interfere with the deposition of fog particles than by promoting adhe- 

 sion, but this is naturally objectionable. 



FIGURE 8. DIAGRAM OF "CRATER." FIGURE 9. DIAGRAM SHOWING CIRCULATION. 



12. Evaporation.- The droplets, originally sharp in outline, become 

 vague and washed on evaporation, doubtless because their curvature decreases, 

 while the area of adhesion remains the same, to the detriment of the nearly 

 spherical curvature at the beginning. Floating globules are usually much more 

 uniform in size and remain more uniform, because the differences due to adhe- 

 sion are absent (see 9,13). It has been stated that when there is evaporation 

 the photograph fails and a blank plate results. It may be assumed that a 

 successful photograph implies as little evaporation of fog particle before the 

 taking of the picture as during this interval. 



13. Floating and moving globules. A final very interesting phenomenon 

 is met with in the case of fog particles floating in a liquid film of oil. It fre- 

 quently happens under these circumstances that there is a sharp line of de- 

 markation in the field of the microscope, probably an edge of contact of the 

 semi-fluid matter on the plate. In all such cases there is apt to be continuous 



