350 C. Barns — Large and Small Coronas. 



of 76 cm corresponding to the volume expansion of about 1*43 . 

 The pressure difference usually applied in the experiments was 

 ^>=31"2, and the volume expansion 1*72. 



3. Alternation of large and, small coronas {periodicity). 

 Data. — The small coronas are usually sharp ; but the large 

 coronas appear blurred and filmy, accompanied with much 

 rain. Remembering that all operations are conducted in a way 

 strictly the same, the annexed figures 1 to 4 shows the coronas 

 seen in the successive exhaustions. The angular diameter or 

 aperture is sin <£/2 = s/60, or nearly cf) — s/S0. The eye at the 

 goniometer was about 40 cm from the axis of the condensation 

 chamber (placed as close as possible to insure clearer vision) 

 and the source of light 250 cm beyond it. Observations were 

 made along the axis of the cylinder, placed horizontally. 



In the case of 2 min. periods between the exhaustions (fig. 1) 

 the periodicity is maintained without exception. For brevity 

 let the smaller coronas be called inferior, the larger coronas 

 superior. Frequently a very small inferior corona I evokes, a 

 relatively large superior corona h, or larger inferior coronas 

 are followed by smaller superior coronas ; but this is not always 

 the case. As a more general rule, if the aperture is interme- 

 diate between the inferior and superior coronas, the succeeding 

 corona is of the same size and oscillation terminates. Similar 

 remarks may be made relative to the diagram, fig. 2, for 

 3 m periods between the exhaustions, or for fig. 4, for the case 

 of dust-free air energized by weak radium (10,000 X) in sealed 

 glass tube. 



4. jRemarks on the results. — It will conduce to clearness to 

 take the increase of apertures, s, with the increase of pressure 

 difference, 8p, first in order. If the exhaustion is insufficient, 

 the groups of smaller nuclei will escape precipitation and the 

 coronas be relatively small. After all nuclei, large and small, 

 are caught, higher sudden exhaustion can no longer increase 

 the apertures. More water is instantaneously precipitated per 

 cubic centimeter. Nevertheless this counter-effect, if it is such, 

 will also vanish with increasing pressure differences, because of 

 the accentuated rapidity, of thermal radiation. The adiabatic 

 method ceases to be effective. Finally the necessity of produc- 

 ing sudden cooling simultaneously with extreme dilatation is a 

 complication ; for in view of the relative slowness of diffusion, 

 it will eventually be impossible to keep the instantaneously 

 dilated water vapor saturated, without arresting the growth of 

 the fog particles. Above Sp— 40 cm the effect of sudden exhaus- 

 tion may be conceived to actually dry the air, seeing that the 

 density of vapor is instantly reduced more than one half, and 

 hence even slight differences of supersaturation at the outset 

 may show themselves effectively at these high exhaustions. 

 Experimentally (figs. 6 and 7), these surmises are not fully 



