IV PREFACE. 



has usually been used as a source of light. Much better results are 

 obtained, however, by the use of the virtually monochromatic mercury 

 lamp as a source. This is sufficiently intense and admits a more definite 

 optical interpretation. Experiments are therefore given in the second 

 part of Chapter II, with a view to standardizing these simplified coronas 

 by the method of successive exhaustions and phosphorus nuclei. 



The third part of the chapter gives an account of further progress made 

 in increasing the efficiency of the fog chamber by reducing its size com- 

 patibly with the use of a new type of goniometer. 



In an endeavor to standardize the coronas in terms of the nucleation 

 involved, by the aid of separate small sealed aluminum tubes containing 

 radium, used singly or in groups, very little progress was made, because 

 the coronal diameter varies as the sixth root of the intensity of ionization. 

 The experiments of Chapter III, however, lead to certain remarkable 

 results on the distribution of ionization with reference to the position 

 within the fog chamber of the sealed aluminum tubelets (beta and gamma 

 rays being in question, largely the latter). 



If the parts of the fog chamber consist of different materials or not, 

 the maximum ionization due to primary and secondary radiation rarely 

 coincides with the position of the radium. In a horizontal cylindrical 

 fog chamber, closed at one end and open for exhaustion at the other, 

 the maximum ionization is found to move from the closed end to the 

 exhaustion end as the radium moves from the closed end to the middle 

 of the chamber. As the radium moves further the maximum remains 

 near the exhaustion end, but the ionization diminishes in marked degree 

 throughout the whole chamber. The ratios of ionization are frequently 

 greater than 2 to i. To obtain maxima of ionization near the middle 

 of the chamber the sealed radium tube must be near the closed end. 

 In other adjustments even minimum ionization was produceable in 

 the middle, as compared with the ends. It appears from the results that 

 it is possible to appreciably displace the ions during the period of ex- 

 haustion, the rate of reproduction being insufficiently rapid as compared 

 with the displacement. 



In a final investigation, Chapter IV, the endeavor is made to stand- 

 ardize the coronas in relation to the number of fog particles represented 

 under given circumstances of exhaustion by aid of Thomson's electron 

 and correlative constants. After a number of trials the first successful 

 method consisted in making a closed aluminum tube, containing an even 

 distribution of radium, the core of a cylindrical condenser, leaded to an 

 inch or more in thickness without. This core was suspended axially from 

 fine wire leading to Dolezalek's electrometer for the measurement of the 

 small voltages and currents involved. The core in question was then 

 removed from the electrical condenser and put into the axis of a dust- 



