DISTRIBUTIONS OF NUCLEI. 49 



fog limit has decreased somewhat further, showing increased abundance 

 of the larger nuclei; but the slope closely resembles the corresponding 

 case for dust-free air. There is no maximum, but a terminal asymptote, 

 or better, a terminal corona of fixed aperture. This corona, which is 

 here the largest obtainable in the fog chamber, is even at the highest 

 ionizations reached at supersaturations below the coronal fog limits 

 for dust-free air, and long before the apparatus has ceased to function. 

 It is impossible to state whether colloidal nuclei are simultaneously 

 present but inactive or whether their inefficiency is due to actual absence. 

 It thus becomes a problem of great theoretical importance in its bearing 

 on this subject to determine how the precipitated supersaturation is 

 distributed among groups of nuclei of different sizes. 



It is finally to be observed that the terminal corona is the same, both 

 for the ionized and the non-ionized state of the gas. The maximum 

 number of nuclei which can be caught per cubic centimeter seems, there- 

 fore, to be a constant for the apparatus and the medium inclosed, and 

 to be independent of the size of the nuclei, whether large like the ions 

 or small like the colloidal nuclei. In the above apparatus, with an air- 

 water medium it is difficult to pass beyond the large green-blue-purple 

 corona. 



43. Radiant fields. The occurrence of a succession of groups of 

 colloidal nuclei of (let us say for simplicity) continuously decreasing size 

 and continuously increasing number, is suggestive; for each group is 

 essential in the given natural but otherwise unknown environment. 

 They are at once restored if withdrawn. These conditions may be imi- 

 tated or varied artificially by approaching the radium tube at different 

 distances from the fog chamber, in which case the efficient nucleation 

 will for weak radiation usually decrease, as the intervening distances 

 are smaller. 



Furthermore, in the presence of radium the character of the phenom- 

 enon is the same, except that the nuclei are throughout larger. With- 

 drawn by precipitation they are at once restored. They are an essential 

 part of air in the new (radiant) environment and the nuclei are again 

 graded. 



It is natural to compare the particular nuclear status introduced in the 

 latter case by a particular kind of radiation (gamma-rays) with a former 

 case of dust-free air in the absence of recognized radiation. In other 

 words, if we abstract from the details of the mechanism which are un- 

 known for the colloidal nuclei, chemical agglomeration might be con- 

 sidered referable to some radiant field, unknown but otherwise essentially 

 alike in kind, to the much coarser nucleations observed on exposure to the 

 known radiant field. The effect of radium, however distant, is virtually 



