PHYSICS — BARUS. 223 



chamber is a most remarkable feature of the exposure. The same, however, 

 is true of the ious. Whether produced by X-rays or by radium acting from 

 long distances, the density of ionization is, as a rule, different in different 

 parts of the fog-chamber, showing the important effects due to secondary 

 radiation. In like manner the change in nucleation produced when the 

 exciting cause (X-ray bulb or radium tube) recedes to different distances 

 from the fog-chamber is of interest. For the non-inclosed bulb the decrease 

 of nucleation is slower than the first power of distance. It is faster for the 

 bulb inclosed in a windowed lead chamber, faster still if the window is closed 

 by a thin tin plate, but in no case does it approach the law of inverse squares. 



Finally, if the rate of decay of the ions can be inferred from the electrical 

 method, 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 the determination not only of the relation of the nuclei 

 corresponding to a given case to two different coronas (caet. par.), but of 

 the absolute nucleation involved as well, not to speak of the possibility of 

 detecting in this way how a given mass of precipitated water is distributed 

 among nuclei of different sizes — one of the most important of the problems 

 outstanding in connection with the subject. 



The volume 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 

 that the colloidal nuclei in a medium of carbon dioxide and water are larger 

 than in the other case, in spite of the presence of the coercible gas in the 

 latter, in which groups of larger molecular aggregates would be anticipated. 

 On the other hand, large colloidal nuclei do seem to occur in the medium of 

 alcohol and air, wherefore it is suggested that the colloidal nuclei of dust- 

 free wet air are primarily to be associated with the saturated vapor, and that 

 the gas enters as a secondary factor. 



Furthermore, a systematic comparison between the number of ions in the 

 atmosphere and the corresponding dust contents in the lapse of time is worked 

 out under the author's direction. No relation 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 contributions to the ioniza- 

 tion of a given region (due to combustion, etc.) vanish so rapidly as to be 

 quite neghgible. Incidentally, the nucleation of the atmosphere of Provi- 

 dence during nearly four years is exhibited. 



Finally the author returns to the inquiry, already begun elsewhere with 

 negative results, whether there is any time change in the colloidal nucleation 

 of dust-free air, such as might be ascribed to the ionization produced by 

 penetrating cosmical radiation coming from without ; for it was to be the 

 plan of these researches to begin with the study of the ordinary dust contents 

 in its dependence on time and thereafter to continue in the same way with 

 the filtered atmosphere. The apparent nucleation of dust-free air is found 



