PREFACE. 



In the following report I have given an account of experiments made 

 with a plug-cock fog chamber during the last year and a half. 



The first chapter summarizes the equations frequently needed and 

 adds other important suggestions relating to the efficiency of the ap- 

 paratus used for condensation of water vapor suspended in air. 



I have adduced, in Chapter II, the results of a long series of experi- 

 ments begun May 9, 1905, to determine whether the colloidal or vapor 

 nucleations of dust-free air show any interpretable variations in the 

 initial regions (ions), which would correspond to variations of a natural 

 radiation entering the chamber from without. The fog-chamber method 

 seems to be too complicated to give trustworthy indications of such 

 changes of ionization as have been since discovered with the aid of the 

 electrical method by Wood and Campbell. An interesting result, how- 

 ever, came out of the experiments in question, as a whole, showing that 

 the vapor nucleation is variable with temperature in the region exam- 

 ined to the extent of about 2 per cent per degree. 



The fog chamber used in the present research having undergone 

 varied modifications since the coronas were last standardized (1904), 

 it seemed necessary to repeat the work for the present report. This 

 was particularly necessary because the subsequent investigations were 

 to depend essentially on the values of the nucleation observed. These 

 comparisons are shown in Chapters III and IV. In the former the 

 diffractions are obtained from a single source of light and the angular 

 diameter of the coronas is measured by a goniometer; in the latter the 

 fiducial annuli of two coronas due to identical sources of light are put 

 in contact and the distance apart of the lamps is measured under known 

 conditions. This contact method has many advantages and above all 

 admits of the use of both eyes. In both cases, moreover, the nucleation 

 of dust-free air, in the presence as well as in the absence of penetrating 

 artificial radiation, is redetermined. All results agree among them- 

 selves and with the older work, as closely as may be expected in work of 

 the present kind, below the middle green-blue-purple corona (usually 

 corresponding to io 5 nuclei); but above this there is much divergence, 

 which will probably not be overcome until some means for keeping the 

 air rigorously homogeneous in nucleation throughout a given series of 

 experiments has been devised. 



Chapter V contains some remarkable results on the properties of 



nuclei obtained from the evaporation of fog particles. It will be seen 



in 



