PREFACE. 



The chief purpose of the present volume is the development of a fog 

 chamber of simplest practical character, capacious enough to admit of 

 the measurement of the largest available coronas, and efficient up to 

 the highest exhaustions applicable; i.e., those which do not uselessly 

 overstep the optical limits of the experiment, where fog particles become 

 so fine as to be virtually inactive in diffracting or scattering white light. 

 This I think has been accomplished, and the results, as far as they go, 

 seem to indicate an efficiency not inferior to Wilson's piston apparatus. 

 I have not, however, been able to get much beyond the large green- 

 blue-purple corona, no matter whether the nuclei selected were effec- 

 tively large, like the ions, or effectively small, like the colloidal nuclei. 

 The forms beyond are flimsy and so nearly colorless as to be useless 

 for measurement; but the steam-jet nevertheless reveals a whole order 

 of axial oranges and yellows, lying beyond, which to my knowledge have 

 not been detected in any form of fog chamber whatever. 



As used in most experiments, including my own earlier work, the fog 

 chamber with a plug stopcock seems to be of very inferior efficiency 

 in comparison with the piston form. This, however, in a properly de- 

 signed apparatus, is the case only when the attempt is made to obtain 

 the isothermal drop in pressure observationally at the fog chamber, 

 closed at once after exhaustion. The datum needed can only be found 

 by computation, and the initial pressures in the fog and vacuum chambers 

 and their final pressure when in contact, always at the same temperature, 

 suffice for this purpose. Though I was prepared for some corrections, 

 I did not anticipate so large a difference between the apparent drop 

 and the true drop of pressure, as actually appears. In the experiments 

 which follow, the ratio is in fact as i,oco to 775, a difference of nearly 

 25 per cent. Hence it will be necessary to restandardize the coronas 

 with this result in view, an undertaking which I hope to begin in the 

 near future. Indeed a large number of incidental results would have 

 made this desirable in the interest of other investigations. For similar 

 reasons I have (as a rule) continued to refer the nucleations of the present 

 volume to the drop in pressure observed at the fog chamber; and such 

 reference is sufficient for the comparisons aimed at, if the same type of 

 apparatus is used throughout, as was the case. 



Having improved the fog chamber to the degree shown in Chapters 

 I and II, it was made use of in Chapter III for certain incidental experi- 



iii 



