C. JBanis — Changes of Colloidal Nucleation. 209 



dependence of the precipitation on changes of temperature of 

 the fog chamber, the average correction* may be taken as 2*3 

 per cent of the values of m at 20° C. Since ?i=6ms 3 /7ra 3 

 approximately (where a is the optical constant of coronas and 

 s their angular diameter on a radius of 30 cm ), for a given s, n 

 varies as m. Therefore n must be increased 2*3 per cent of 

 its value per degree of temperature of the fog chamber above 

 20° C. In this way the corrected data of the table were 

 found. Both are shown in fig. 3. 



The table also contains the data for the corresponding aver- 

 ages of temperature, barometric pressure and ionization, and 

 all data have been further given in the graphs figure 3, with 

 the times (abscissas) laid off on a smaller scale to bring out the 

 relative variations. It is again apparent that no relation of 

 the nucleation curve to the barometer curve or to the ioniza- 

 tion curve can be made out. On the other hand, the colloidal 

 nucleations of the dust-free wet air in the fog chamber agree 

 very fully with the cotemporaneous variations of the tempera- 

 ture of the fog chamber (not of the temperature of the atmos- 

 pheric air without, of which they are also independent). It is 

 even possible to make out the rate at which efficient nuclei 

 are produced when the temperature of the fog chamber 

 increases. Taking the mean trend of both curves (nuclei and 

 temperature), it appears that nearly 8000 colloidal nuclei are 

 generated (apparently), in dust-free wet air, by a rise of tem- 

 perature of one degree centigrade. 



10. Cause of the temperature effect. — In the above experi- 

 ments the nucleations were compared at a fixed value, "335, of 

 the variable {Bp—^ir—ir^j/^p — ir). If, however, the corre- 

 sponding value of the relative drop Bp/p (which assumes that 

 all the water vapor is expanded adiabatically without condensa- 

 tion) be computed, the latter will vary with temperature in a 

 way correlative with the vapor pressures contained in the 

 former. Hence the nucleations computed for this particular 

 series of values of Bp/p will also vary, and the rate was found 

 to be about 6000 nuclei per degree. This is so near the tem- 

 perature eifect given in paragraph 9 that there must be a com- 

 mon cause underlying both. 



I have not yet completed the details of a comparison of the 

 above nucleations in the lapse of time for a fixed value of the 

 drop of pressure Bp/p, but from what has been stated it appears 

 probable that no interpretable variation within the reach of 

 the fog chamber will appear. In relation to Bp/p moreover, 

 * Smithsonian Contrib., No. 1651, p. 135, 1905. 



Am. Jour. Sci.— Fourth Series, Vol. XXIII, No. 135.— March, 1907. 

 15 



