SUMMARY. 103 



of the persistent nuclei produced by relatively intense X-radiation in the 

 lapse of time, showing results similar to Chapter I, sections 14, 15. 



In the same way the occurrence of water nuclei in the presence of 

 colloidal nuclei or of ions was clearly exhibited. These are due to the 

 evaporation of small fog particles, until the increase of vapor pressure 

 due to surface curvature is balanced by the decreased vapor pressure due 

 to some independent phenomenon. Hence if the exhaustions proceed 

 with a slightly opened filter cock, such nuclei must be present in greater 

 number as the evaporation is faster, because increasingly more particles 

 evaporate than subside. 



The result in the first place is a necessary alteration of aperture of 

 successive coronas, all other conditions remaining the same, since the 

 large ones evaporate more fog particles than those of lower order of size 

 (periodicity of coronal diameter). In connection with this phenomenon 

 it is particularly interesting to observe that not only the colloidal nuclei 

 but the ions are available. Questions arise as to whether the fog par- 

 ticles of positive and of negative ions evaporate to like water nuclei, 

 what becomes of the charges, since it is not clear that they should be 

 dissipated, what phenomena arise from the decreased mobility of the 

 loaded ions, etc. 



The means developed in Chapter II were again applied to an effect 

 produced (distance effect), when the radiating source is removed more 

 and more from the fog chamber. The results are necessarily crude ; but 

 they show that in case of pervious wood fog chambers the nucleations 

 decrease more slowly than the first power of distance; they decrease 

 faster for the glass fog chamber; faster still when the X-ray bulb is 

 inclosed in a windowed lead case; fastest when the window is closed 

 with a thin tin plate; but in no case do they reach the law of inverse 

 squares. All this points clearly to the importance of secondary radiation 

 in producing nuclei. The radiation arises both near the bulb and near 

 the fog chamber, as well as in the region between. The number of nuclei 

 produced is therefore dependent upon an integral extended over the 

 whole interior surface of the room and throughout the intervening air. 



In case of exposure to weak radium (10 mg. io.oooX) the asymptote 

 of dust-free air is not reached until the intervening distance is above 150 

 cm.; and it is far from appearing after 1.5 cm. of lead have been pene- 

 trated. 



The occurrence of minima of nucleation, when such weak radium is 

 carried from a long distance quite up to the fog chamber, is again strik- 

 ingly brought out, provided the drop of pressure is sufficiently above the 

 fog limit of dust-free air. It is also observed when the ions are produced 



