IV PREFACE. 



ments. I have already shown in case of dust-free air and the persistent 

 nuclei produced by intense X-radiation that the distribution of nuclei 

 within the fog chamber is a most remarkable feature of the experi- 

 ment. The same, however, is true of the ions. Whether produced by 

 X-rays acting from long distances or by radium, the density of ioniza- 

 tion is as a rule very different in different parts of the fog chamber, 

 showing the important effects due to the presence of secondary radiation 

 within. Again, the change of nucleation produced when the exciting 

 cause (X-ray bulb or radium tube) is removed at different distances 

 from the fog chamber was to be reinvestigated. Associated with this 

 experiment is the occurrence of minima of nucleation for certain dis- 

 tances, supposing the exhaustion to be sufficiently high to induce con- 

 densation of ions and colloidal nuclei in presence of each other. Finally, 

 if the rate of decay of ions can be inferred from independent electrical 

 experiments, a method for the standardization of coronas is presented 

 which bids fair to be the most satisfactory solution of the problem 

 suggested. The method admits of a determination not only of the rela- 

 tion of the nuclei corresponding, in a given case, to two different coronas 

 {cat. par.), but of the absolute nucleation involved. There is also a 

 possibility of detecting in this way how a given mass of precipitated 

 water is distributed among nuclei of different sizes when occurring 

 together one of the most important of the problems outstanding in 

 connection with this apparatus. 



Chapter IV adduces a variety of results for colloidal nuclei in media 

 other than air-water. It is shown, for instance, that there is no evidence 

 to prove the colloidal nuclei in a medium of carbon dioxide and water 

 are larger than in the normal case of air and water, in spite of the 

 presence of the coercible gas in which groups of larger molecular aggre- 

 gates would be anticipated. On the other hand relatively large colloidal 

 nuclei do seem to occur in a medium of air and alcohol vapor. Thus 

 it is suggested that colloidal nuclei in dust-free wet air are to be associated 

 with the saturated vapor and that the gas is only secondarily involved. 



In Chapter V, undertaken by Miss L. B. Joslin under my direction, a 

 systematic comparison is worked out of the relations between the num- 

 ber of ions in the atmosphere and the corresponding dust contents in 

 the lapse of time. No direct connection is apparent, whence it follows 

 that as the nucleation is largely of local origin, other sources must be 

 looked to for the ionization, or that the enormous local output of ions 

 from an industrial community vanishes so rapidly as to be quite negli- 

 gible. On the other hand, it may be possible to detect evidences of 

 "absorption" in the curves obtained. Incidentally the nucleation of 

 the atmosphere of Providence during nearly four years is exhibited. 



