FLEETING NUCLEI. 133 



around its state of chemical equilibrium, the sense of the departure 

 being in the long run as frequently positive as negative. 



These are the chief features of the hypothesis by which I have been 

 guided in my work. It will now be desirable to present an outline 

 of such facts bearing on the whole question as occur in my own 

 researches. To obtain sufficiently varied conditions it will manifestly 

 be necessary to produce nuclei in the condensation chamber (briefly 

 called the fog chamber) itself, without introducing foreign material. 

 This may be done by aid of the X-rays or the beta and gamma rays of 

 radium, preferably to the exclusion of the emanation and even of the 

 alpha rays. Changes of uucleation so obtained may be regarded as 

 arising out of the dust-free contents of the fog chamber. 



90. Notation. The nucleations, N, referred to in these paragraphs, 

 are computed from the angular radius, </>, of the corona seen in the 

 fog chamber and the quantity of water precipitated in the given exhaus- 

 tion, in the way detailed in my report to the Smithsonian Institiition 

 (1905). In the figures, which for brevity are referred to in place of the 

 tables, D denotes the distance of the X-ray bulb or the radium tube 

 (of thin aluminum, hermetically sealed) from the near end of the fog 

 chamber. Exp. is the time of exposure in minutes to the radiation 

 stated ; Lp, the lapse of time after exposure to the radiation ceases, 

 or the time during which the nuclei under observation are left without 

 interference. The abscissas (unless otherwise specified) frequently 

 indicate the number of an observation ; i. e., they are merely distribu- 

 tive and represent the character of successive phenomena to the eye. 

 The fog chambers used were sometimes rectangular boxes of wood 

 impregnated with wax and provided with plate-glass windows ; at 

 other times clear glass cylinders, 15 cm. long and 15 cm. or more in 

 diameter. The latter case insures greater freedom from traces of 

 leakage, but coronal aperture, 2 <f>, must be measured parallel to the 

 axis of the cylinder. Usually the chord, s, for a radius of 30 cm. 

 will be given, so that 2 sin < = si 30. 



Exhaustions are preferably made at a pressure difference (S/>) 

 between the outside and inside of the fog chamber, just below the point 

 (to be called fog limit 8p ) at which dust-free nonenergized saturated 

 air condenses without foreign nuclei. 8^ depends on the particular 

 apparatus used. 



91. Fleeting nuclei Ions. Let the X-radiation to which the dust- 

 free air is exposed be relatively weak, so that the density of ionization 

 may remain below a certain critical value. The nuclei observed on 

 condensation are then very small, and they require a high order 



