PREFACE 



The present meinoii' is .-i coiitinuiitioii ul' my work of 1893 (U. 8. Weather 

 Bureau, Bulletin 12), and with less al)ru[)t transition, of the ExperiimnU with Ion- 

 ized Air, published iu ^Smithsonian Contributions to Knowledge, Vol. XXIX, in 1901. 

 Several outstanding questions in the latter volume have been answered. 

 Thus in Chapter I, the nucleation and the ionization emanating from ]ihos[)hoius 

 are contrasted in their relation to temperature. The results which a[)pear, viz., 

 that the nucleation begins at a definite temperature (about l;}") with a maximum 

 of intensity while the ionization increases more gradually to reach a maximum 

 later at about 20"; moi'eover that moisture, while unfavorable to nucleation, pro- 

 motes ionization ; these results explain much of the baffling variability of the 

 phosphoi'us ionizer with which in the volume cited, and particularly in the case of 

 experiments with spherical condensers, I made vain endeavors to contend. The 

 reason why the i-ate of electrical leakage and the rate at which nucleated air flows 

 throu"h a tubular condenser are linear functions of each other but with a different 

 coefficient in different series of experiments, is also apparent with other results of 

 minor interest. 



Having determined to approach the question of nucleation by a new method 

 in this volume, viz., through the measurement of the coronas' of cloud particles, it 

 seemed imperative to undertake a detailed investigation of the whole i)heuomenon 

 of coionas, at the outset, insofar as it would contribute to these experiments. By 

 no other means, to my knowledge, is the magnificence of coronal disjilay so ap- 

 proachable in its entii-ety, as when evoked by cloud particles of identical diameter, 

 variable at i)leasure. The colors of cloudy condensation may be classified into 

 three groups: The first includes the normal coi'onas (as I shall call them) due to 

 relatively large particles. They are all chai'acterized by a centi'al white disc and 

 there are no axial colors. The second class includes coronas due to relatively small 

 particles; the centi-al disc is colored, and as the particles increase in diameter from 

 •a lower limit, the central colors follow the ordei- of Newton's interferences. The 

 third group comprises the axial colors seen in the undeviated ray and due to some 

 kind of harmonic absorption stii)erimposed on the diffractions of the first two 

 groups. These coloi'S are increasingly saturated as the cloud particles decrease in 



' I regret that all my attemiits to obtain Kiessling's work on Ddmmerungserschcinungen were 

 unsuccessful. 



